In this article on Earned Value Reporting we'll look at the Schedule Performance Index. The Schedule Performance Index refers to how the project is performing in terms of actually following the project schedule.
Much of the following information came from Newell and Grashina's book entitled 'The Project Management Question and Answer Book.'
What is the schedule performance index?
The schedule performance index is a measure of how well the project is doing in terms of following the project schedule. It is a comparison of the project tasks that were planned to be accomplished to the work that was really accomplished. The index is a value that allows projects of different sizes to be compared.
The schedule performance index is like the schedule variance discussed previously with one important difference. When we calculated the schedule variance, the result was a figure in dollars. If the dollars were negative, the variance was considered bad, and if the dollars were positive, the variance was considered good. The problem with this method is that it is difficult to compare projects of different size to one another. It would be better to have a measure that gives the health of the project regardless of its size. For this purpose we will use indexes.
Instead of subtracting the budgeted cost of work scheduled from the budgeted cost of work performed, as we did when we calculated the schedule variance, we will divide the same two numbers.
SPI = BCWP / BCWS
SPI = EV / PV
We can see that if the project is following its plan, the amount of work accomplished and the amount of money spent to accomplish it are the same, and the resulting value will be one. So, an index of one means that the project is following its plan.
If the budgeted cost of work scheduled is greater than what is being accomplished, the denominator in the fraction will be larger than the numerator, and the resulting value will be less than one. This is a bad condition. If the budgeted cost of work scheduled is less than what is being accomplished, the resulting number will be greater than one and this is considered good.
Example:
A project is two weeks behind schedule at the time of the calculation. The project has fifteen people working full-time. Assume that each person costs $1,000 per week. BCWS at this point in the project is $500,000. What is the schedule performance index?
The project is two weeks behind schedule and there are fifteen people working full-time on the project. This results in being behind schedule by thirty person-weeks or $30,000.
SPI = BCWP / BCWS
The BCWP is $500,000 ? $30,000 = $470,000.
The BCWS is $500,000.
SV = BCWP - BCWS = -$30,000
SPI = BCWP / BCWS = $470,000 / $500,000 = 0.940
Notice that a smaller project such as one that had a BCWS of $50,000 and a BCWP of $47,000 would also have a schedule performance index of 0.940. Again, this helps the project manager who is managing different parts of a project in which the sizes of the parts are different. The schedule performance index, like the cost performance index, indicates the health of the project regardless of its size.