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

Transcription

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