Using cellphone data to measure population movements. Experimental analysis following the 22 February 2011 Christchurch earthquake

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1 Using cellphone data to measure population movements Experimental analysis following the 22 February 2011 Christchurch earthquake

2 Crown copyright This work is licensed under the Creative Commons Attribution 3.0 New Zealand licence. You are free to copy, distribute, and adapt the work, as long as you attribute the work to Statistics NZ and abide by the other licence terms. Please note you may not use any departmental or governmental emblem, logo, or coat of arms in any way that infringes any provision of the Flags, Emblems, and Names Protection Act Use the wording 'Statistics New Zealand' in your attribution, not the Statistics NZ logo. Liability While all care and diligence has been used in processing, analysing, and extracting data and information in this publication, Statistics New Zealand gives no warranty it is error free and will not be liable for any loss or damage suffered by the use directly, or indirectly, of the information in this publication. Citation Statistics New Zealand (2012). Using cellphone data to measure population movements. Wellington: Statistics New Zealand ISBN (online) Published in February 2012 by Statistics New Zealand Tatauranga Aotearoa Wellington, New Zealand Contact Statistics New Zealand Information Centre: Phone toll-free Phone international

3 Information about the data 'Christchurch cellphones' The main dataset used for analysis in this report is a subset of cellphones known as Christchurch cellphones. These are anonymised cellphones that initiated calls from Christchurch cellphone towers only, during the 10 days before the 22 February 2011 earthquake in Christchurch. Percentage of cellphones initiating calls The percentage of cellphones initiating calls on any given day is the proportion of cellphones in the 'Christchurch cellphones' subset that initiated one or several voice calls (or text messages) on the day. Source For the purposes of this report, Statistics New Zealand analysed a subset of daily counts of cellphone transmissions registered by New Zealand telecommunications companies during time periods in 2010 and

4 Contents 1 Purpose of this report Summary of results Introduction... 8 Why information on population movements is useful... 8 Using cellphone data to collect migration information Monitoring people movements following the 22 February 2011 Christchurch earthquake Cellphone data is very useful in an emergency situation People increased weekends away from Christchurch following the 22 February earthquake Highest level of return movements were from districts in Canterbury Destination patterns from Christchurch to other regions were largely unchanged compared with Lowest populated regions of the South Island were popular destinations Otago and Auckland were the most frequently visited regions outside Canterbury People relocating to the North Island had the slowest return to Christchurch Nearby recreationally attractive areas were important destinations Nearby and highly populated districts attracted many people from Christchurch By the end of April inflows to Christchurch were similar to the previous year Monitoring people movements using different subsets of cellphones Using different subsets of cellphones affected observed levels of movements within the region Using data from SMS messaging resulted in a higher level of observed movements Evaluation of the usefulness of cellphone data Options for further analysis of cellphone data Conclusions Technical notes Data, methods, and geographies used Confidentiality Caveats Methodology used References and further reading References Further reading Appendix Privacy Impact Assessment

5 List of tables and figures Tables by chapter 4 Monitoring people movements following the 22 February 2011 Christchurch earthquake Christchurch cellphones initiating voice calls, for selected broad geographic areas Evaluation of the usefulness of cellphone data Information useful for monitoring short-term population movements Figures by chapter 4 Monitoring people movements following the 22 February 2011 Christchurch earthquake Percentage of Christchurch cellphones initiating voice calls during 24-hour periods 10 2 Voice calls initiated by Christchurch cellphones within Christchurch city during 24- hour periods Percentage of Christchurch cellphones initiating voice calls from selected regions outside Canterbury Percentage of Christchurch cellphones initiating voice calls from regions outside Canterbury Ratio of resident population to Christchurch cellphones initiating voice calls, for selected regions outside Canterbury Percentage of Christchurch cellphones initiating voice calls from selected regions during 24-hour periods Percentage of Christchurch cellphones initiating voice calls during 24-hour periods from Canterbury region and other regions (excl. Christchurch city) Percentage of Christchurch cellphones initiating voice calls during 24-hour periods from South Island (excl. Canterbury) and the North Island Distribution of Canterbury region's resident population, for districts in the Canterbury region Percentage of Christchurch cellphones initiating voice calls from districts in Canterbury region Percentage of Christchurch cellphones initiating voice calls from districts in the Canterbury region (excl. Christchurch city) Percentage of Christchurch cellphones initiating voice calls during 24-hour periods, for selected districts in the Canterbury region Percentage of non-christchurch cellphones initiating voice calls from Christchurch city on Fridays Monitoring people movements using different subsets of cellphones Percentage of each cellphone subset initiating voice calls during 24-hour periods from districts in the Canterbury region Percentage of each cellphone subset initiating voice calls during 24-hour periods from regions outside Canterbury Percentage of SMS subset and voice subset cellphones initiating texts or voice calls during 24-hour periods

6 1 Purpose of this report This report examines the use of cellphone data to monitor short-term population movements. It contains examples of analysis that can be undertaken with data records of cellphone usage during and after a natural disaster, and discusses the strengths and weaknesses of this data. The analysis was prompted by the Canterbury earthquakes of 2010 and It focuses on population movements within the region, and to other regions. This report answers the following questions: What are the movement patterns of people who relocated following the 22 February 2011 earthquake in Christchurch city? Which areas attracted the highest percentages of people? By when had most people re-located and by when had most people returned? Are results sensitive to the method used to select subsets of cellphones for the purposes of tracking movements? What are the quality constraints of cellphone data? The results in this report are taken from our initial investigations of these questions. 6

7 2 Summary of results Our analysis showed that cellphone data can be a very useful data source for tracking population movements following a disaster. It allows us to see which geographical areas attract high percentages of people, patterns of return movements over time, and flows of non-residents into the emergency zone. However, cellphone data is not able to show us which residential areas people leave from, which areas people relocate to following an event, or the actual number of people who relocate, temporarily or permanently. We also found that using data from different subsets of cellphones did not alter the resulting movement patterns. This indicates an acceptable degree of robustness in the main results. The movement patterns of a subset of Christchurch cellphones following the 22 February 2011 earthquake show: Otago and Auckland were the most frequently visited regions outside Canterbury following the quake more people visited or temporarily relocated to regions outside Canterbury than to districts within the Canterbury region from the second week onwards people mainly relocated to nearby districts in the Canterbury region during the first week after the quake and to other regions during the second week the weekends following the quake, and the Easter holiday, showed much larger numbers of people moving temporarily to areas outside Christchurch city than those observed the previous year most people had returned to Christchurch by about five weeks following the quake people who visited or temporarily relocated to the North Island returned to Christchurch at the slowest rate by the end of April, inflows of visitors and new migrants to Christchurch from other areas of the country resumed. 7

8 3 Introduction Why information on population movements is useful After a natural disaster, emergency services, councils, and other national and regional authorities need information on internal population movements to assess where people move to, as rapid movements of large numbers of people to smaller areas can put pressure on a range of local services and businesses. Increased time spent travelling can also have implications for main transport routes and transport services. Information about people movements can determine where aid is deployed and where temporary service sites should be located. This is important immediately after an event, for example if temporary accommodation is acquired, as well as later, when more stable short-term accommodation needs are settled. Information about people movements is also needed to measure the number of people returning to the area, the number who stay away, and inflows of new migrants. This information is needed until movement stabilises and measures such as the Census of Population and Dwellings provide more detailed information on regional population changes. Research questions arising from this need for information are as follows: Which regions/cities/districts receive the greatest percentages of relocated people? What are the movement patterns over time? How permanent are movements? What are the usual patterns of movement compared with those made as a result of the emergency event? Using cellphone data to collect migration information Cellphone data has not yet been widely explored as a useful data source to monitor aggregated movements of individuals following a major event. Following the Haiti earthquake in January 2010, a group of researchers analysed changes in locations of all cellphones in Haiti before and after the quake. By tracking population displacement the study revealed that the data source has potential to track people movements following large-scale natural disasters (Bengtsson, Lu, Garfield, Thorson, & von Schreeb, 2010). After the Christchurch earthquake on 22 February 2011, the Ministry of Civil Defence and Emergency Management approached Statistics New Zealand for information to enable emergency response and planning. This query led to an investigation of the potential of cellphone data and other administrative data sources. From a dataset of anonymised daily registrations of voice calls and locations of associated cellphone towers, we analysed aggregate movements of a subset of Christchurch-based cellphones. We compared the patterns of movements out of Christchurch city (to other areas of New Zealand) with movements over the same time and space under normal circumstances. For more detail on how we derived our results, see chapter 7. We also looked at cellphone data s strengths (such as the ability to update movement patterns quickly) and weaknesses (incomplete population coverage and lack of demographic, socio-economic and usual residence information about those who move). 8

9 Statistics NZ undertook this analysis with the cooperation of contributing telecommunication companies. We also worked with the Ministry of Civil Defence and Emergency Management, and other responding agencies, to understand the types of information needs that cellphone data may respond to. Our analysis and the results presented in this report are a step towards understanding the potential of cellphone data while acknowledging a changing information technology environment. All results are from our initial investigations. 9

10 4 Monitoring people movements following the 22 February 2011 Christchurch earthquake This section presents results from analysis of the movements of subsets of cellphones in the weeks immediately following the 22 February 2011 earthquake. The results cover cellphone use in an emergency situation, distribution of calls made from Christchurch and from other areas on any given day, patterns of movements from Christchurch city to other regions of New Zealand and to other areas within the Canterbury region, and movements into Christchurch city of cellphones visiting from other areas. Cellphone data is very useful in an emergency situation For the purposes of this study, we used a subset of cellphones called 'Christchurch cellphones'. These are cellphones that initiated calls from cellphone towers within Christchurch city only, during the 10 days before the quake. Chapter 7 describes the dataset in more detail. In general, cellphone voice calls show a regular calling pattern across the week. Temporal changes in calling pattern reflect occurrences of specific events such as public holidays. During usual working days (Monday to Friday) the likelihood of making voice calls is high, and during weekends this likelihood reduces. During the weekend, when business calling is reduced and when friends and families are more likely to spend time together, the number of cellphone voice calls is much reduced. Minimal usage is observed on Sundays. Figure 1 1 Percentage of Christchurch cellphones initiating voice calls during 24-hour periods Percentage of Christchurch cellphones (1) initiating voice calls during 24-hour periods 1 February 30 April 2010 and Percent 22nd February earthquake Earthquake Memorial Day Easter February March April 1. Christchurch cellphones includes cellphones that initiated voice calls from cellphone towers within Christchurch city only, during the 10 days before the 22 February earthquake (ie February 2011). Source: Statistics New Zealand 10

11 Figure 1 shows that the 22 February Christchurch earthquake resulted in increased cellphone usage, both at the time and during the weeks following the disaster. Compared with the number of voice calls during the same time period in 2010, the daily calling rate increased significantly in the weeks following the quake. However, the same weekly regularity in voice call pattern remained largely unchanged. People increased weekends away from Christchurch following the 22 February earthquake The highest percentages of Christchurch cellphones making calls from areas outside Christchurch were observed for weekends immediately following the quake, ie starting 26 February and 5 March. Around 20 percent of Christchurch cellphones were making calls from areas outside Christchurch on these two weekends. Similarly, the long weekend created by Earthquake Memorial Day, on Friday, 18 March 2011, prompted many to visit areas outside Christchurch. Movements of Christchurch cellphones were monitored daily in the weeks following the earthquake by observing the percentage of cellphones that continued to make calls from Christchurch compared with the percentage that made calls from areas outside Christchurch. As expected, for the cellphones making calls on a given day, increasing percentages of Christchurch cellphones moved to areas outside Christchurch city during the first three days (23 25 February 2011) leading up to the weekend. Figure 2 2 Voice calls initiated by Christchurch cellphones within Christchurch city during 24-hour periods Voice calls initiated by Christchurch cellphones (1) within Christchurch city during 24-hour periods 1 February 30 April 2010 and Percent February March April 1. Christchurch cellphones includes cellphones that initiated voice calls from cellphone towers within Christchurch city only, during the 10 days before the 22 February earthquake (ie February 2011). Note: Percentages are based on the Christchurch cellphones that made calls from one territorial authority only for any 24-hour period between 1 February and 30 April. Christchurch cellphones that did not initiate calls or made calls from more than one territorial authority for the 24-hour period were excluded. Source: Statistics New Zealand By early April, the percentage of cellphones initiating calls from Christchurch had returned to being close to the 2010 level, indicating that most people who relocated had returned to Christchurch. However, due to the fact that the Easter public holiday period fell later in 2011 than in 2010, and the limited time period observed, it was not possible to estimate the percentage of Christchurch cellphones that had returned to mainly making calls from Christchurch. 11

12 Highest level of return movements were from districts in Canterbury Between 22 February and the end of March 2011, the same percentage of Christchurch cellphones initiated calls from Christchurch city compared with the same period in 2010, but there were large increases in the percentages that initiated calls from districts in the Canterbury region (table 1). From data observations we can infer an increased level of return movements to the city from other areas, mainly during weekends (see figure 2 for the decreased percentages of daily calls from areas within Christchurch city). This may reflect people arranging a secondary residence with access to basic amenities in close proximity to Christchurch and being able to regularly travel between there and their original residence. Table 1 1 Christchurch cellphones initiating voice calls, for selected broad geographic areas Christchurch cellphones initiating voice calls (1) For selected broad geographic areas 22 February 31 March 2010 and 2011 Geographic area Percentage of Christchurch cellphones initiating calls Percent Districts in the Canterbury region (excl. Christchurch city) South Island (excl. Canterbury region) Regions in the North Island New Zealand (excl. Canterbury region) Christchurch city Any area Geographic area Average number of days on which Christchurch cellphones initiated calls Districts in the Canterbury region (excl. Christchurch city) New Zealand (excl. Canterbury region) Christchurch city Christchurch cellphones making calls during special events (eg Red Cross fund-raising event in Dunedin on 2 March 2011) were excluded. Source: Statistics NZ Days On average, people initiated calls from areas outside Christchurch city on more days following the earthquake than in the same period in However, possibly due to a high level of return movements there were only moderate increases. For the same reason the average number of days on which calls were made from Christchurch city were only slightly higher in 2010 (16.2 days) compared with 2011 (15.8 days). 12

13 Destination patterns from Christchurch to other regions were largely unchanged compared with 2010 Movements of Christchurch cellphones to other regions over the six-week period following the 22 February earthquake followed the same trends as the previous year. However, outflows were more concentrated to the Otago region; about 9 percent of Christchurch cellphones visited this region. Excluding any special events this was more than double the rate in the same six-week period in Figure 3 3 Percentage of Christchurch cellphones initiating voice calls from selected regions outside Canterbury Percentage of Christchurch cellphones (1) initiating voice calls from selected regions outside Canterbury 22 February 31 March 2010 and 2011 Region Auckland Waikato Bay of Plenty Manawatu-Wanganui Wellington Tasman Nelson Marlborough West Coast Otago Southland Percent 1. Christchurch cellphones includes cellphones that initiated voice calls from cellphone towers within Christchurch city only, during the 10 days before the 22 February earthquake (ie February 2011). Notes: Calls made by Christchurch cellphones during days of special events (eg Red Cross fund-raising event in Dunedin on 2 March 2011) were excluded. The Easter periods April 2011 and 2 5 April 2010 were therefore excluded from this analysis. Figures for Northland, Gisborne, Taranaki, and Hawkes Bay regions were excluded from the graph due to small proportions of Christchurch cellphones making calls from these regions. Cellphones can make calls from several districts during a time period, so percentages of calls made from areas cannot be summed. Source: Statistics New Zealand 13

14 Figure 4 4 Percentage of Christchurch cellphones initiating voice calls from regions outside Canterbury Lowest populated regions of the South Island were popular destinations Low-populated regions close to Canterbury attracted the highest percentages of Christchurch cellphones after the 22 February quake. In relation to population size, other low-populated regions in the northern South Island also attracted high percentages of Christchurch cellphones. Histories of established social connections between Christchurch residents and people living in low-populated regions of the South Island were likely to be a motivating factor for many to visit these regions. 14

15 Figure 5 5 Ratio of resident population to Christchurch cellphones initiating voice calls, for selected regions outside Canterbury Ratio of resident population to Christchurch cellphones (1) initiating voice calls For selected regions outside Canterbury 22 February 31 March 2011 Region Auckland Waikato Bay of Plenty Manawatu-Wanganui Wellington Tasman Nelson Marlborough West Coast Otago Southland Ratio 1. Christchurch cellphones includes cellphones that initiated voice calls from cellphone towers within Christchurch city only, during the 10 days before the 22 February earthquake (ie February 2011). Notes: Ratios have been derived as the percentage of Christchurch cellphones making calls from the region divided by the percentage of the New Zealand population resident in the region. Percentages of Christchurch cellphones making calls from the region are the figures presented in figure 3 and are also included in the categories displayed in figure 4. Percentages of the New Zealand population resident in the region were derived from provisional estimates at 30 June Figures for Northland, Gisborne, Taranaki, and Hawke s Bay regions were excluded from the graph due to small percentages of Christchurch cellphones making calls from these regions. Source: Statistics New Zealand The Auckland region, with about one-third of the New Zealand resident population, was visited by a relatively low percentage of Christchurch cellphones between the 22 February earthquake and the end of March. However, the lowest-populated regions of the South Island West Coast and Marlborough, with less than 2 percent of the New Zealand population attracted relatively the highest percentages of Christchurch cellphones during that time. 15

16 Otago and Auckland were the most frequently visited regions outside Canterbury People relocated mainly to areas close to Christchurch city during the first week following the 22 February quake, specifically districts in the Canterbury region. They also relocated to other more populated regions in the South Island and the North Island. Figure 6 6 Percentage of Christchurch cellphones initiating voice calls from selected regions during 24-hour periods Percentage of Christchurch cellphones (1) initiating voice calls from selected regions during 24-hour periods By number of weeks since 1 February 1 February 30 April 2010 and Christchurch cellphones includes cellphones that initiated voice calls from cellphone towers within Christchurch city only, during the 10 days before the 22 February earthquake (ie February 2011). Notes: Percentages are based on the total number of Christchurch cellphones making calls from the region during 24-hour periods averaged over the week days Monday to Friday. Special events that attracted large numbers of temporary visits to the region have been treated as outliers and calls made on these days have been excluded from the calculations. Cellphones can make calls from more than one region during a time period, so percentages of calls made from areas cannot be summed. Relocation to areas outside the Canterbury region generally happened at a later stage. Another highly populated region in the South Island, Otago, attracted 2.4 percent of Christchurch cellphones during the second week following the quake. Other lesspopulated regions in the South Island attracted much smaller percentages of Christchurch cellphones. Christchurch cellphones initiating calls from the Auckland and Wellington regions indicated that the number of people relocating to these regions from Christchurch peaked during the second week following the earthquake, at 1.4 and 1.0 percent, respectively. 16

17 People relocating to the North Island had the slowest return to Christchurch People who relocated to North Island regions had a slower rate of return to Christchurch city compared with people who relocated to South Island regions outside Canterbury. From about three weeks following the 22 February quake there were higher percentages of Christchurch cellphones continuing to make calls from North Island regions compared with cellphones making calls from South Island regions excluding Canterbury (figure 8). Proximity to Christchurch after relocating, as well as the cost and complexity of return travel, may have affected people s rate of return. However, the data cannot tell us whether people who moved took the likelihood of return into account when selecting their destinations. We can see that those who moved further away generally spent longer periods away. Figure 7 7 Percentage of Christchurch cellphones initiating voice calls during 24-hour periods from Canterbury region and other regions (excl. Christchurch city) Percentage of Christchurch cellphones (1) initiating voice calls during 24-hour periods from Canterbury region and other regions (excl. Christchurch city) By number of weeks since 1 February 1 February 30 April 2010 and Percent Number of weeks since 1 February 2011 New Zealand excl. Canterbury region 2011 Canterbury region excl. Christchurch city 2010 Canterbury region excl. Christchurch city 2010 New Zealand excl. Canterbury region 1. Christchurch cellphones includes cellphones that initiated voice calls from cellphone towers within Christchurch city only, during the 10 days before the 22 February earthquake (ie February 2011). Notes: Percentages are based on total number of Christchurch cellphones making calls from the region during 24-hour periods averaged over the weekdays Monday to Friday. Special events that attracted large numbers of temporary visits to the region have been treated as outliers and calls made on these days have been excluded from the calculations. Cellphones can make calls from more than one region during a time period, so percentages of calls made from areas cannot be summed. Source: Statistics New Zealand 17

18 Figure 8 8 Percentage of Christchurch cellphones initiating voice calls during 24-hour periods from South Island (excl. Canterbury) and the North Island Percentage of Christchurch cellphones (1) initiating voice calls during 24-hour periods from South Island (excl. Canterbury) and the North Island By number of weeks since 1 February 1 February 30 April 2010 and Percent Number of weeks since 1 February 2011 South Island excl. Canterbury region 2011 North Island 1. Christchurch cellphones includes cellphones that initiated voice calls from cellphone towers within Christchurch city only, during the 10 days before the 22 February earthquake (ie February 2011). Notes: Percentages are based on total number of Christchurch cellphones making calls from the region during 24-hour periods averaged over the weekdays Monday to Friday. Special events that attracted large numbers of temporary visits to the region have been treated as outliers and calls made on these days have been excluded from the calculations. Cellphones can make calls from more than one region during a time period, so percentages of calls made from areas cannot be summed. Source: Statistics New Zealand From the second week following the quake and onwards, percentages of Christchurch cellphones that made calls from areas outside the Canterbury region were higher than the percentage that made calls from areas outside Christchurch city but within the Canterbury region. The average percentage making calls from regions outside Canterbury peaked at 7.6 percent during the second week following the quake (figure 7). This result comprised 4.4 percent making calls from the South Island excluding Canterbury region and 3.6 percent from the North Island, as presented in figure 8. Christchurch cellphones making calls from districts in the Canterbury region averaged 7.4 percent during the first week following the quake (figure 7). 18

19 Nearby recreationally attractive areas were important destinations Two low-populated north Canterbury districts, Kaikoura and Hurunui, attracted relatively higher percentages of Christchurch cellphones than the highly populated Waimakariri and Selwyn districts, which are closer to Christchurch city. Following the quake, many Christchurch residents might have been attracted by the outdoor recreational facilities, for example Hanmer Springs, that these two districts offer. Figure 9 9 Distribution of Canterbury region's resident population, for districts in the Canterbury region Distribution of Canterbury region's resident population For districts in the Canterbury region At 30 June 2011 Kaikoura District Hurunui Waimakariri Selwyn Ashburton Timaru Mackenzie Waimate Percent Notes: The percentage of Canterbury region's population resident in Christchurch city was estimated at 65.8 percent at 30 June Percentage distribution of the Canterbury region's resident population by territorial authority (district) was derived from provisional estimates at 30 June Source: Statistics New Zealand In 2011, less than 3 percent of Canterbury s population was living in Kaikoura and Hurunui districts (figure 9), but 2.8 and 9.4 percent, respectively, of Christchurch cellphones made calls from these areas between the earthquake and the end of March (figure 10). In comparison, Waimakariri and Selwyn comprised 16 percent of Canterbury s population, and around 19 percent of Christchurch cellphones made calls from each of these districts during the same period. 19

20 Figure Percentage of Christchurch cellphones initiating voice calls from districts in Canterbury region Percentage of Christchurch cellphones (1) initiating voice calls from districts in Canterbury region 22 February 31 March 2010 and 2011 Kaikoura District Hurunui Waimakariri Selwyn Ashburton Timaru Mackenzie Waimate Percent 1. Christchurch cellphones includes cellphones that initiated voice calls from cellphone towers within Christchurch city only, during the 10 days before the 22 February earthquake (ie February 2011). Notes: Calls made by Christchurch cellphones during days of special events (eg Earthquake Memorial Day on 18 March 2011) were excluded. The Easter periods April 2011 and 2 5 April 2010 were therefore excluded from this analysis. Cellphones can make calls from several districts during a time period, so percentages of calls made from areas cannot be summed. Source: Statistics New Zealand Overall, between the 22 February earthquake and the end of March nearly half of Christchurch cellphones (44.9 percent) had made calls from the districts in the Canterbury region; this compared with 28.1 percent over the same time period in However, relative movement patterns to the districts by Christchurch cellphones in 2011 remained unchanged following the emergency situation compared with Selwyn district, which is the third most populated district in Canterbury, attracted the largest percentage of Christchurch cellphones (nearly 20 percent) following the earthquake. The second most populated district, Timaru, only attracted 6.5 percent of Christchurch cellphones, possibly because it is much further from Christchurch city. 20

21 Figure Percentage of Christchurch cellphones initiating voice calls from districts in the Canterbury region (excl. Christchurch city) 21

22 Nearby and highly populated districts attracted many people from Christchurch The highly populated districts of Waimakariri and Selwyn, both in the Canterbury region and near Christchurch city, were the most frequently visited areas during the first weeks following the 22 February quake. In the first week, 2.5 percent of Christchurch cellphones made calls from Waimakariri, and 2.3 percent made calls from Selwyn. These two districts are the usual areas for Christchurch residents to relocate to, maybe temporarily, and this pattern is indicated by the 2010 data in figure 10. In 2011, there were still increased percentages of Christchurch cellphones making calls from these areas toward the end of April, or about 10 weeks following the earthquake. Figure Percentage of Christchurch cellphones initiating voice calls during 24-hour periods, for selected districts in the Canterbury region Percentage of Christchurch cellphones (1) initiating voice calls during 24-hour periods For selected districts in the Canterbury region By number of weeks since 1 February (1 February 30 April 2010 and 2011) 1. Christchurch cellphones includes cellphones that initiated voice calls from cellphone towers within Christchurch city only, during the 10 days before the 22 February earthquake (ie February 2011). Notes: Percentages are based on total number of Christchurch cellphones making calls from the region during 24-hour periods averaged over the weekdays Monday to Friday. Special events that attracted large numbers of temporary visits to the region have been treated as outliers and calls made on these days have been excluded from the calculations. Cellphones can make calls from several districts during a time period, so percentages of calls made from areas cannot be summed. Two South Canterbury districts attracted similar percentages of Christchurch cellphones during the first week following the quake 1.1 percent of calls were made from Ashburton and 0.8 percent made calls from Timaru. Hurunui district in North Canterbury is a common holiday destination for Christchurch residents, and the Easter periods in 2011 and 2010 saw an increased number of Christchurch cellphones making calls from the district. 22

23 By the end of April inflows to Christchurch were similar to the previous year Immediately following an emergency situation there is an increased inflow of visitors to the area. These people may be involved with emergency services or visiting to support friends and family. This rapid inflow is then followed by reduced levels of short-term visitors from other areas of the country compared with normal times. To monitor inflows to Christchurch we used a subset of cellphones that made calls from areas outside Christchurch city only, during the 10 days before the quake ( non- Christchurch cellphones ). This data indicated that toward the end of April the number of people visiting Christchurch city from other areas of New Zealand was similar to 2010 levels. Percentages of non-christchurch cellphones initiating calls on Fridays were observed, which is the day of the week with the highest likelihood of calls being initiated (figure 13). Figure Percentage of non-christchurch cellphones initiating voice calls from Christchurch city on Fridays Percentage (1) of non-christchurch cellphones (2) initiating voice calls from Christchurch city on Fridays 1 February 30 April 2010 and Percent Easter Friday Easter Friday Earthquake Memorial Day Number of Fridays since 1 February 1. Highest percentages of cellphones making calls from Christchurch city by visitors were observed on Fridays. 2. Non-Christchurch cellphones are a subset of cellphones that initiated voice calls from cellphone towers outside Christchurch city only, during the 10 days before the 22 February earthquake (ie February 2011), and then made calls in Christchurch after 22 February. Source: Statistics New Zealand 23

24 5 Monitoring people movements using different subsets of cellphones This section presents some additional findings that contribute to the robustness of the results presented in chapter 4. These results cover consistency of movement patterns using different subsets of cellphones, and show whether movement patterns differ when observing a subset of SMS messages. Using different subsets of cellphones affected observed levels of movements within the region The results presented in chapter 4 were based on a subset of cellphone IDs that made calls from Christchurch city only, during the 10 days before the 22 February quake. Using other subsets of cellphones revealed no differences in movement patterns, but indicated increases in relative levels of movements to the districts in the Canterbury region. Future use of cellphone data will require more detailed selection of the population of analysis. During any 10-day period many people will travel out of and back to Christchurch city and maybe initiate calls from nearby districts or from regions further away. So it is important to examine whether results of people movements are sensitive to changes in call activity by subsets of cellphones from areas outside the city. We compared results from Christchurch cellphones (cellphones with all calls made from Christchurch city in the 10 days before the quake) with two other subsets of cellphones. Subset 1 included cellphones that made at least 90 percent of their calls from Christchurch city in the 10 days before the quake, and subset 2 included those that made at least 80 percent of their calls from the city in the same period. 24

25 Figure Percentage of each cellphone subset initiating voice calls during 24-hour periods from districts in the Canterbury region Percentage of each cellphone subset initiating voice calls during 24-hour periods from districts in the Canterbury region By number of weeks since 1 February 1 February 30 April Percent Subset 2 Subset 1 Christchurch cellphones Source: Statistics New Zealand Number of weeks since 1 February Notes: Christchurch cellphones includes cellphones that initiated voice calls from cellphone towers within Christchurch city only, during the 10 days before the 22 February earthquake (ie February 2011). 'Subset 1 includes cellphones that initiated at least 90 percent of their voice calls from cellphone towers within Christchurch city, during the 10 days before the 22 February earthquake (ie February 2011). Subset 2 includes cellphones that initiated at least 80 percent of their voice calls from cellphone towers within Christchurch city, during the 10 days before the 22 February earthquake (ie February 2011). Percentages are based on total number of Christchurch cellphones making calls from the region during 24-hour periods averaged over the weekdays Monday to Friday. Special events that attracted large numbers of temporary visits to the region have been treated as outliers and calls made on those days have been excluded from the calculations. 25

26 Figure Percentage of each cellphone subset initiating voice calls during 24-hour periods from regions outside Canterbury Percentage of each cellphone subset initiating voice calls during 24-hour periods from regions outside Canterbury By number of weeks since 1 February 1 February 30 April Percent Subset 2 Subset 1 Christchurch cellphones Number of weeks since1 February Notes: Christchurch cellphones includes cellphones that initiated voice calls from cellphone towers within Christchurch city only, during the 10 days before the 22 February earthquake (ie February 2011). 'Subset 1 includes cellphones that initiated at least 90 percent of their voice calls from cellphone towers within Christchurch city, during the 10 days before the 22 February earthquake (ie February 2011). Subset 2 includes cellphones that initiated at least 80 percent of their voice calls from cellphone towers within Christchurch city, during the 10 days before the 22 February earthquake (ie February 2011). Percentages are based on total number of Christchurch cellphones making calls from the region during 24-hour periods averaged over the weekdays Monday to Friday. Special events that attracted large numbers of temporary visits to the region have been treated as outliers and calls made on those days have been excluded from the calculations. Source: Statistics New Zealand From calls made by subsets 1 and 2 we can see that people were more likely to have travelled to nearby districts in the Canterbury region than to other regions during the 10- day period February Cells that made calls from districts in Canterbury before the quake were more likely to continue visiting these areas following the quake (figure 14). For subset 2, the percentage of movements to regions outside Canterbury following the quake was only marginally larger than that of the original Christchurch cellphones subset (figure 15). By contrast, the percentage of cellphones that relocated to other areas of Canterbury during the week following the quake was 10.3 percent for subset 2, compared with 7.4 percent for the Christchurch cellphones subset. 26

27 Using data from SMS messaging resulted in a higher level of observed movements Cellphones are more frequently used for short message service (SMS) messaging (or texting ) than for voice calls. Use of SMS data therefore enables a greater number of observed cellphone movements. For the purposes of this study, a subset of cellphones initiating SMS messages and/or voice calls was available for the month of June The movements of a subset of cellphones that initiated SMS messages from Christchurch city only, during the 10 days before the 13 June 2011 quake ('SMS subset') were compared with an equivalent subset that initiated voice calls during that period ('voice subset'). This analysis indicated that higher percentages had visited districts in the Canterbury region during the time from the 13 June quake to the end of June. The percentage of the SMS subset initiating messages from districts in the Canterbury region during June 2011 was 18.3 percent. Based on the voice subset initiating calls during the same time period, a much lower percentage appeared to have moved (13.4 percent). However, we observed very similar levels of activity between the two subsets from regions outside Canterbury during that time (8.1 percent of the SMS subset and 8.0 percent of the voice subset). Figure Percentage of SMS subset and voice subset cellphones initiating texts or voice calls during 24-hour periods Percentage of SMS subset (1) and voice subset (2) cellphones initiating texts or voice calls during 24-hour periods 1 30 June Percent SMS subset Voice subset June 2011 month 1. SMS subset includes cellphones that initiated SMS messages from Christchurch city only, during the 10 days before the 13 June earthquake (ie 3 12 June 2011). 2. Voice subset includes cellphones that initiated voice calls from Christchurch city only, during the 10 days before the 13 June earthquake (ie 3 12 June 2011). Note: Percentages of the voice subset making calls are not directly comparable with results in figure 1. Source: Statistics New Zealand Generally, people are about twice as likely to initiate SMS messages compared with voice calls. Observing the call pattern of the SMS and voice subsets, respectively, revealed that at a time of emergency, for example the 13 June 2011 earthquake in Christchurch city, people use voice calling more than SMS messaging. However, the weekly regularity of call patterns for the two types of messaging is very similar (figure 16). 27

28 6 Evaluation of the usefulness of cellphone data A subset of cellphone records for the time period before and after an emergency event allows identification of spatial patterns of short-term population movements that differ from normal. Indicators of the amount of short-term population displacement following the 22 February 2011 earthquake in Christchurch city were also investigated using Ministry of Education re-enrolment data for primary, intermediate, and secondary school students, and New Zealand Post relocation data. Reports publishing results from these two data sources provided indicators of the geographic distribution of people who relocated over a time period. (See Re-enrolment of Christchurch school students and Household relocation in Canterbury region on the Statistics NZ earthquake information portal Table 2 2 Information useful for monitoring short-term population movements Information useful for monitoring short-term population movements For selected data sources Information useful for monitoring movements School reenrolment data (1) Information provided by data NZ Post relocation data (2) Cellphone data Population movements between observed points in time Number of students who have re-enrolled at primary, intermediate, or secondary schools Number of households (and people in households) that notified NZ Post of relocation Movements of individuals as cellphone calls are transmitted to nearest cellphone towers Demographic and other Age, sex Age groups less than 16 years or other, sex, tenure Permanence of relocation Time of return to original area of residence Geographic area of origin and destination of residence Not directly available from data source but from other surveys of personal use of cellphones Not available Available Not available Limited to time of reenrolment at original school Limited to geographic area of school Available Available Not directly available but some approximation is possible Limited to geographic area of cellphone towers where calls were initiated 1. Ministry of Education re-enrolment data for primary, intermediate, and secondary school students was published by Statistics NZ (2011), with the latest update at 10 August Examination of NZ Post relocation data was undertaken by OPUS (2011) for the period before the 22 February earthquake (11 January February 2011) and for the period following the earthquake (23 February April 2011). 28

29 New Zealand Post relocation data has information that is most directly related to shortterm population movements. Data records include households (and associated people) who notify of relocation, some of their characteristics, and the timing, permanence, and geographic details of original address and area of relocation. Similarly, Ministry of Education school re-enrolments provide information on the movements of school-aged children between two points in time. In contrast, cellphone records allow continued monitoring of people-based movements over time in any geographical context. Comparing the variable content available for these three data sources highlights some core limitations of cellphone data. This data lacks direct information on the permanence of relocation and the timing of return to the original area of residence, and it is also missing demographic and usual residence characteristics of the people relocating. These limitations will motivate emergency services to seek information from a number of data sources. Options for further analysis of cellphone data The focus of this report has been to show how cellphone data has the potential to meet a number of identified information needs for emergency management planning. Other types of information gaps at times of natural disasters can potentially be filled by different analysis of the data. Options include undertaking extended analysis of cellphone data to examine: the changes in intensity of cellphone use over time the differences in spatial 'mappings' of individuals' trajectories before and after the 22 February 2011 earthquake. Cellphone data may also have applications not directly linked to emergency management, for example when estimating population changes in urban spaces for given time periods. Detailed discussion of this is beyond the scope of this report, but some potential uses are: to estimate the population that uses location-based services, for example peak tourist populations in the Queenstown urban area to estimate the population that uses associated infrastructures and facilities at the time of a planned major event, compared with at other times to assist urban planning in general (eg to estimate the population using space and infrastructure, population density in urban spaces, and aggregated urban population movements over time and space) to assist with estimation of internal movement of people between aggregated geographic areas, over a defined time period. Conclusions This report has responded to a number of specific research questions that were raised to provide necessary information at a time of civil emergency. A subset of cellphone IDs was selected ( Christchurch cellphones ) for the purposes of tracking movements following the 22 February 2011 Christchurch earthquake. Using different subsets did not alter the movement patterns indicating an acceptable degree of robustness in the main results. SMS data showed a greater degree of movement within the region, but movement patterns to other regions remained the same. This indicates that better population coverage will be gained by including records of SMS but analysis of voice calls will determine movement patterns adequately. The analysis highlighted the main strengths of the data source for the purposes of emergency management. Cellphone data allows us to 29

30 monitor changes in population movement patterns and changes in the presence of cellphone users in urban-spatial contexts. Further, it allows us to continuously track urban movements over time an attribute unique to cellphone data. Cellphone data can provide the following information for emergency management: Which regions, districts, and cities attract significantly higher inflows of people from an area of emergency over time. Patterns of rapid movements over space and time following an event, including return movements. Relative inflows of people to a broadly defined area of emergency that are significantly different to expected levels of inflow. Which regions, cities, and districts potentially have increased population making calls from their area due to an emergency event in a nearby region. Cellphone data cannot sufficiently provide the following information: Which residential areas have high percentages of people leave after an emergency. Which areas of residence people relocate to following an event. The actual number of people who relocate, temporarily or permanently. 30

31 7 Technical notes Data, methods, and geographies used Parameters of the data Cellphone transmissions include voice calls and SMS messages (or texts ). These are registered by where and when they are made to a resolution given by the location of the nearest cellphone tower. For the purposes of this study, data records included the time a voice call or SMS message was initiated and the location of the first cellphone tower it connected to, by cellphone ID. The main subset of cellphone IDs used to track movements following the quake was selected to ensure that users were most likely to be residents of Christchurch city, but it also may have included some visitors. Included in the subset were cellphone IDs that initiated calls from Christchurch city cellphone towers only, during the 10 days before the quake, (referred to as Christchurch cellphones in this report). Christchurch cellphones: A subset of cellphones that initiated voice calls from cellphone towers within Christchurch city only, during the 10 days before the 22 February earthquake (ie February 2011). While SMS messages generate a much larger volume of data (because they are initiated approximately twice as often as voice calls) they are not currently retained by all companies. For this reason, the results in this report are derived primarily from observing movements of a subset of cellphones making voice calls. However, a subset of cellphones sending SMS messages was also analysed to determine whether patterns of movements based on this source gave substantively different results to those based on voice calls (see definitions for these subsets in the section 'Using data from SMS messaging resulted in a higher level of observed movements' of chapter 5). Data records supplied by New Zealand cellphone companies included: a unique cellphone identifier, created by the mobile phone company, for each domestic cellphone number (cellphone ID) to maintain user anonymity date of call the identifier for the cellphone tower from which calls were initiated a count of the voice calls or SMS messages by cellphone identifier and cellphone tower for the 24-hour period. Basic method used Patterns of short-term movements over time were derived by observing changes in the percentage of Christchurch cellphones that initiated calls in an area within a 24-hour period (for more detail, see Methodology used in this chapter). It has been assumed that the percentage of Christchurch cellphones making calls from a geographic area is proportional to the number of Christchurch residents present in the area with personal use of a cellphone. 31

32 Geographic units included For the purposes of this study, locations of cellphone tower addresses have been coded to the standard geographic classifications for regional councils (regions) and territorial authorities (cities and districts). Analysis of cellphone movements to finer geographic levels, for example for area units of Christchurch city, has been omitted from this report. Note that only calls registered by cellphone towers with a permanent address have been included. Frequency of cellphone use among New Zealanders The most recent Household Use of Information and Communication Technology survey (Statistics NZ, 2009) recorded that overall 85 percent of the population aged 15 years or older had personal use of a mobile phone during some or all of the last 12 months. Highest rates of usage were observed for ages 15 to 44 years (93 percent) and lowest rates of usage were recorded among people aged 75+ (42 percent). It can be assumed that coverage is weak for the very young and the very old. Confidentiality Publication of business administrative data collections are guided by the principles and requirements of Statistics NZ policies. There are clear guidelines for publishing information from these collections while protecting the confidentiality of the businesses that contribute to them. This report adheres to those policies and guidelines. It aims to outline the most useful information available from a sample of anonymised cellphone records while maintaining full confidentiality of individual cellphone companies and individual cellphone numbers. A privacy impact assessment was undertaken for this project (see the appendix). Caveats Generally, calls recorded by cellphone companies imply a number of caveats that may influence observed data summaries. No distinction can be made between business and personal cellphones or calls. In some cases individuals will operate more than one cellphone. During any observed timeframe, new cellphone IDs may enter the dataset, some cellphone IDs may transfer to another company, and other cellphone IDs may cease to exist. Some cellphone towers may be non-operable for short time periods and cellphone access may be restricted for some people during a civil emergency. Records of cellphone calls are not linked to demographic and socio-economic characteristics of the users. Therefore, identification of the factors that pushed people to relocate or not to relocate is not possible. For the purposes of this study, the intensity or number of calls initiated during a given time period has not been analysed; although the data is available it was not needed for this study. Similarly, the numbers of times cellphone users visited locations have been excluded. This is the first time Statistics NZ has used cellphone data records to derive patterns of population movements in New Zealand. We acknowledge that if people using cellphones have different movement patterns to those with no personal use of cellphones then results will be biased. However, we are assuming that short-term movement patterns shown by the subsets of cellphone IDs are broadly representative of movement patterns for the Christchurch city population as a whole following the February earthquake. 32

33 Methodology used For the purposes of this study, a subset of active cellphones, α ij, includes the cellphone IDs that initiated at least one call during a 24-hour period, i', and in a geographic area, j'. The geographic area contains a number of cellphone towers with identifiable coordinate information. Hence, for any observed time period (of say n days) a unique subset of active cellphones in a geographic area is A j = n i=1 a ij. A subset of cellphones used to track movements following the quake was selected to ensure that users were likely to be residents of Christchurch city. Cell IDs initiating calls from Christchurch cellphone towers (j = Christchurch city) only, during the 10 days before the quake (i = [12 February 21 February]) were included in the sample (A j is in this case referred to as Christchurch cellphones ). Proportioning the number of active cellphones for any 24-hour period to the total number of Christchurch cellphones following the quake as i a ij / A j, enables the monitoring of relative changes in cellphone activity for 24-hour time periods ( i ) and in geographic areas ( j ). Patterns of short-term movements over time have been derived by observing changes in the percentages of Christchurch cellphones that initiated calls from geographic areas before and after the quake. Percentages of Christchurch cellphones initiating calls from geographic areas during 24- hour periods before and after the quake have been averaged for Monday to Friday of each week between 1 February and 30 April. The average smoothing of percentages of Christchurch cellphones initiating calls ensures confidentiality of the cellphone users for the observed time period. 33

34 References and further reading References Bengtsson, L, Lu, X, Garfield, R, Thorson, A, & von Schreeb, J (2011). Internal population displacement in Haiti. Karolinska Institute, Center for Disaster Medicine, and Columbia University, Schools of Nursing and Public Health. Retrieved 11 January 2012 from OPUS (2011). Preliminary report on household relocation from and within the Canterbury region 22 February April Retrieved 11 January 2012 from Statistics New Zealand (2009). Household Use of Information and Communication Technology: Available from Statistics New Zealand (2011). Re-enrolment of Christchurch school students. Retrieved 11 January 2012 from Further reading Song, C, Qu, Z, Blumm, N, & Barabasi, A (2010). Limits of predictability in human mobility. Science 327(5968), Available from Love, T (2011). Population movement after natural disasters: a literature review and assessment of Christchurch data. Sapere Research Group. Available from Statistics New Zealand (2011). Evaluation of alternative data sources for population estimates. Wellington: Statistics New Zealand. Available from Pulselli, R, Romano, P, Magaudda, S, & Tiezzi, E (2008). Monitoring human mobility in urban systems: a new technique based on cell phone activity. Proceedings REAL CORP 008. Available from Pulselli, R, Romano, P, Ratti, C, & Tiezzi, E (2008). Computing urban mobile landscapes through monitoring population density based on cell-phone chatting. International Journal of Design & Nature and Ecodynamics 3(2),

35 Appendix Privacy Impact Assessment The results in this report were derived from a feasibility study, for which a Privacy Impact Assessment has been prepared in accordance with Statistics NZ s confidentiality protocols. In accordance with the Office of the Privacy Commissioner, Statistics NZ has established appropriate safeguards and processes to govern data transferred from commercial organisations. The risks to individual privacy arising from projects using cellphone data have been identified and processes developed to ensure that privacy is not jeopardised. Statistics NZ is required to comply with the confidentiality provisions of the Statistics Act 1975 and also Security in the Government Sector protocols. The Statistics Act 1975 regulates the collection of information, whether from statistical surveys or administrative records, for use in producing official statistics by government departments. The Act includes strict provisions to protect the security of collected information and to prevent the release of identifiable information about an individual or business. Section 37 of the Act states that information provided under the Act shall only be used for statistical purposes. In accordance with the Act, Statistics NZ ensures that information is kept securely, access is restricted and any publication avoids disclosing identifiable information. All data held by Statistics NZ is subject to these strict provisions. The Privacy Act 1993 provides protection to information about an individual and applies to every agency that deals with personal information. The Act (information privacy principle 3) generally requires that agencies collecting personal information directly from individuals ensure that those individuals are aware of the purposes of collection, but an exemption is made for information that will be used for statistical or research purposes which will not be published in a form that could reasonably be expected to identify the individual concerned. The Privacy Impact Assessment found that the feasibility study: complies with the provisions of the Statistics Act 1975, particularly by ensuring that anonymised cellphone records transferred to Statistics NZ are kept securely, access is restricted (on a 'need to know basis), and any publication avoids disclosing identifiable information complies with the provisions of the Privacy Act 1993, ensuring that privacy concerns of individuals and businesses are addressed and that the data will be accessed for statistical or research purposes only complies with Statistics NZ s well-established policies, procedures, and systems to ensure adequate measures of physical and electronic security, which includes a requirement to delete all records from Statistics NZ s IT systems at an agreed time does result in additional privacy risks, particularly during the period in which authorised personnel are processing data transferred from individual cellphone companies to Statistics NZ, but these risks have been managed to the lowest possible level involves the potential risk of negative public perception appropriate consultation and the openness, appropriate use, and transparency of the processes followed have mitigated this risk, and credible responses to public concerns are available if required. 35