Earned Value Management

What might you do late or early?
What does the SPI show that the Cost Performance Index reflects the difference between the amount of work that has been finished and the amount of work that was expected to be finished?
Depends on the relative size of what?

Transcription

1 Earned Value Management Allen D. Holliday February 27, 2008 Computer Science Department California State University, Fullerton Cost and schedule are two of the primary concerns of a project manager, who wants to know: How much money has been spent? (Will we finish within budget?) How much work has been completed? (Will we finish on time?) There s a very important characteristic of the two measures, cost and schedule they are interrelated and shouldn t be examined independently. One measure can be good while the other is bad. Being ahead of schedule or under budget isn t necessarily good news. You may have spent too much to get ahead of schedule, or you may be under budget because you don t have the staff to get enough work done, and you ll going to finish late (and may also overspend). What you need is a method of measuring spending and progress that allows you to judge their combined effect. Earned Value Management is such a method. It can be applied continually to assess a project to see if it s proceeding as expected tasks are being completed on schedule and the rate of spending is within budget. When deviations occur, you can determine how serious they are and what corrective action is required. Definitions There are three measures involved in earned value calculation: Earned Value (EV): the value of the work actually completed, in terms of the work s budgeted cost. Planned Value (PV): the value of the work expected to be completed, also in terms of the work s budgeted cost. Actual Cost (AC): the money that s been spent. We use one unit (money) for all three measures so that we can calculate unit-free ratios. There s an important point about a term used in these definitions the measure of work. This isn t just some subjective judgment of how far along a task is, such as we feel that we re 35% done. It has to be an objective measure of something physical that can be measured or counted, a milestone or a deliverable. The 35% is acceptable if it means that you ve finished 35 of 100 tasks. (Or delivered 35 of 100 products, or finished 35 of 100 parts of a system, and so forth.) In older versions of Earned Value Management, Earned Value was called Budgeted Cost of Work Performed (BCWP), Planned Value was called Budgeted Cost of Work Expected (BCWE) and Actual Cost was called Actual Cost of Work Performed (ACWP). The current terms are shorter and clearer. 1

2 Performance Indexes Two earned value performance indexes are calculated by the formulas shown below: Cost Performance Index (CPI) reflects the difference between the planned cost of the work that s been completed and the actual cost of that work, regardless of whether the work is on time, early, or late. It s calculated as follows: CPI EV AC If you re right on budget, the index will be 1.0. If you re spending less than you d planned for a given amount of work, AC will be less than EV and CPI will be greater than 1.0. If you re overspending, AC will be greater than EV and CPI will be less than 1.0. Schedule Performance Index (SPI) reflects the difference between the amount of work that s been finished and the amount of work that was expected to be finished, regardless of the cost of that work. It s calculated as follows: SPI EV PV If you re right on schedule, the index will be 1.0. If you re getting more done than you d planned, EV will be greater than PV and SPI will be greater than 1.0. If you re behind schedule, EV will be less than PV and CPI will be less than

3 There are nine possible combinations of schedule and cost indexes, some good and some bad, summarized below. Current Situation shows the meaning at a given point, Final Result shows what will happen if the current trend remains unchanged. Condition Current Situation Final Result On budget, on schedule On budget, ahead of schedule On budget, behind schedule Over budget, on schedule Over budget, ahead of schedule Over budget, behind schedule Under budget, on schedule Under budget, ahead of schedule Under budget, behind schedule Neutral. Good; you re working faster and getting more for your money. Mixed; the work is late, but you haven t spent too much so far. Mixed (some good, some bad); getting the work done on time, but spending too much. Mixed; the work is early, but you re spending faster. Bad news; the work is late, and you re spending too much. Good; getting the work done on time, and cheaper than expected. The best news; you re getting more work done at a lower cost. Mixed; the work is late, but you re also spending slower. You ll finish right on time and on budget. You ll finish early and you won t spend all of your budget. You ll finish late and overrun your budget by spending at your planned rate for too long. You ll finish on time, but overrun your budget. Depends on the relative size of the two indexes. If you re just as ahead of schedule as you are over budget, you ll finish early and you ll spend exactly what you d planned. If you re less ahead of schedule than you re over budget, you ll finish early but still overspend. If you re farther ahead of schedule than you re over budget, you ll finish early and underspend (because your over budget problem will end early). You ll finish late and overrun your budget. You ll finish on time and underspend your budget. You ll finish early with more money left over than the Ahead of schedule, on budget case. Depends on the relative size of the measures: If you re just as behind schedule as you are under budget, you ll be late but you ll spend exactly what you d planned. If you re less behind schedule than you re under budget, you ll finish late but underspend. If you re farther behind schedule than you re under budget, you ll finish late and overspend (because your slow spending rate will continue for too long and the final total will be more than you d planned). 3

4 In reality, schedule predictions aren t as simple and straightforward as the previous table shows. SPI is a summary of the amount of work completed. It doesn t account for the parallel nature of the project s tasks. Some of those tasks can be delayed without affecting the finish date of the project. SPI could be greater than 1.0 at some point and be equal (or even less than) 1.0 at the finish. Cost predictions aren t affected by this parallelism money spent can t be recovered in the same manner a schedule slip on a non-critical task can. Once CPI indicates a spending overrun, you must either increase productivity or eliminate work to finish early. Both of these are difficult to achieve. SPI and CPI are indicators showing you whether your project is performing according to your plan. You must decide how serious the problem is and what you need to do about it. Earned Value Management provides the trigger for problem investigation; you must define the solution. 4

5 Example An example will help. Imagine that you have a project with 10 tasks. To make the arithmetic simple, we ll make them all one month long and cost $100 each. The project should take ten months to complete and cost $1000. We ll be looking at the situation halfway through the project right after month five of a ten-month schedule. The budget and schedule plan calls for one half of the budget to be spent and one half of the work to be performed at this point. These linear rates aren t the typical cost or schedule curves of a real project, but they will make the example easier to follow. Also, the deviations of cost and schedule are the same in this example, while in a real project, this probably won t occur. One deviation will be larger than the other for instance, a large budget overrun and a small schedule gain. The diagrams below show a schedule bar chart and a spending graph for the ten tasks. Remember, one of these charts by itself isn t enough to show the complete picture of how you re doing. Spending Graph The spending graph also shows three conditions; whether they are good or bad depends on how they relate to the schedule chart. It might be OK to overspend if you re going to finish early and stop spending soon. Underspending might be a problem, if you re also behind schedule and will have to spend money for a longer time than planned, eventually overrunning your budget. If we view the spending rates of these three conditions, the On Budget rate is $100 per month, the Over Budget rate is $140 per month, and the Under Budget rate is $60 per month. A high rate may mean that you have a larger staff than you d originally planned, or that salaries are higher than you d expected. A low rate may mean a smaller staff than planned, or lower salaries than expected. 5

6 Schedule Bar Chart The schedule bar chart s gray boxes indicate completed tasks; the bars show three possible conditions. The Behind Schedule bar shows that we ve only finished the work that we planned to spend $300 on, but we re at a time when we should have finished $500 s worth. The On Schedule bar shows that we ve finished that $500 s worth of work. The Ahead of Schedule bar shows that we ve finished $700 s worth of work at a time when we d planned to finish only $500 s worth. If we view the task completion rates of these three conditions, the On Schedule rate is one task per month, the Behind Schedule rate is only 0.6 tasks per month, and the Ahead of Schedule rate is 1.4 tasks per month. Cost and Schedule at Completion The at completion values are calculated by: 1. Final cost Budget / CPI 2. Final duration Planned duration / SPI The final cost is a reflection of both how fast we re spending and how long we ll be spending. Remember that5 these examples are simple linear cases that illustrate a point but won t be encountered on actual projects. 6

7 Calculations of Performance Indexes Here are the calculations for the nine combinations of CPI SPI values. The labels reflect what the schedule bar chart and spending graph show individually; the calculated status values tell a more complete story when they re examined together as noted below. On Budget, On Schedule CPI AC SPI 1.0 Everything looks fine here, and it really is. You re right on plan for both cost and schedule. On Budget, Ahead Of Schedule CPI AC SPI 1.4 The cost status is better than staying even. Although you re spending at the rate you d planned, you re getting more work done for the money than you had expected. You ll finish early and spend less than you d planned, because your spending at your planned rate will end early. Note that the CPI is not 1.0, even though the AC value by itself seems to show the project being right on budget half the budget spent in half the schedule. The higher CPI is because the EV is high; more than half of the work has been performed. This is also reflected in the higher SPI. $1000 / 1.4 $ months / months 7

8 On Budget, Behind Schedule CPI AC SPI 0.6 The CPI shows that, even though you re spending at the rate you d planned, you re not getting enough work done for the money. The SPI shows that you re going to finish late. In the end, you ll spend more than you d planned. Although you won t spend faster than you d planned, you ll be spending longer than you d planned. Note that again the CPI is not 1.0, even though the AC value by itself seems to show the project being right on budget half the budget spent in half the schedule. The lower index is because the BCWP is low; less than half of the work has been performed. $1000 / 0.6 $ months / months Over Budget, On Schedule CPI AC SPI 1.0 The CPI shows that you re spending too much money, while the schedule status shows that you re going to finish on time. You ll spend more than you d planned. $1000 / 0.71 $ months / months 8

9 Over Budget, Ahead Of Schedule CPI AC SPI 1.4 The CPI shows that, even though you re spending more than you d planned, you re getting more work done for that money than you d planned. If things continue at the same pace, you ll finish early and spend exactly what you d planned. Note that the CPI is 1.0, even though the AC value by itself shows the project being over budget more than half the budget spent in half the schedule. The 1.0 value is because the EV is equally high; more than half of the work has been performed. $1000 / 1.0 $ months / months Over Budget, Behind Schedule CPI AC SPI 0.60 The CPI shows that things are pretty bad, much worse than the other two Over Budget cases. The SPI shows that you re going to finish late. You ll be seriously overspent, because you ll be spending both faster and longer than planned. $1000 / 0.43 $ months / months 9

10 Under Budget, On Schedule CPI AC SPI 1.0 A good situation. You re not spending as fast as you d planned and you re not late. You ll finish on time and you won t spend as much as you d planned. $1000 / 1.67 $ months / months Under Budget, Ahead Of Schedule CPI AC SPI 1.4 The best of all possible cases. You won t spend as much as you d planned and you re going to finish early. $1000 / 2.33 $ months / months 10

11 Under Budget, Behind Schedule CPI AC SPI 0.6 The CPI by itself doesn t show a problem; it says that you ve spent just as much on the work performed as you d planned. The SPI shows the true reason for the underspending not enough work being done, which is a problem. In the end, you ll finish late but you won t spend more than you d planned. Even though you ll be spending longer than you d planned, you ll spend slower than you d planned, Note that the CPI is 1.0, even though the AC value by itself shows the project being under budget less than half the budget spent in half the schedule. The 1.0 value is because the EV is equally low; less than half of the work has been performed. $1000 / 1.0 $ months / months 11