A parameter of Earned Value Analysis. The ratio of actual performance to planned performance.  It is generated by taking the sum of the budgets of those activities (or milestones) completed by the data date (Budgeted Cost for Work Performed {BCWS} or Earned Value {EV}) and dividing by the sum of the budgets of all activities scheduled to have been completed by that date (Budgeted Cost for Work Scheduled {BCWS} or Planned Value {PV}).

A ratio of less than 1.0 suggests that the project is being performed more slowly than was planned (that is, it is behind programme); a ratio of greater than 1.0 indicates the project is being performed faster than planned (that is, it is ahead of programme).

However, since the SPI is unrelated to critical path analysis, it can lead to a distorted picture of the progress of the schedule, and even result in work being performed out of order specifically in order to "game" the SPI.  The duration of the project is dependent on the performance of activities on the Critical Path; yet completion of activities that are off the critical path and have lots of Total Float can have equal or more Earned Value weight (depending on their budgets) as compared to activities on the CP. As a result, project managers will sometimes deliberately seek to inflate the SPI by performing big budget activities that have float, while temporarily ignoring the all-important critical path activities whose delay will extend the project duration. 

The above problem can be corrected by:

1.  Using a separate baseline for the SPI than for the Cost Performance Index, namely, one that places the activities/milestones for the SPI baseline on the Backward Pass of the critical path schedule, where no activities have float and where any delay in any activity's dates will delay the project duration; and

2.  Refusing to give credit for earned value to any activity/milestone performed earlier than the period in which it was scheduled to be performed.