Schedule Performance Index (acronym "SPI") is a measure of efficiency of a project schedule to determine how well this schedule is performed in terms of work initially planned and work actually done. The measure is used by project managers and leaders on a regular basis to assess schedule progress by calculating the ratio of work physically performed versus work scheduled, with regard to approved budget.
The measure of SPI is used in earn value analysis in projects to explore any cost-related problems and their impact to scheduled project activities and jobs. It is calculated by the following general formula:
SPI = --------------
BCWP means Budgeted Cost of Work Performed
BCWS means Budgeted Cost of Work Scheduled
The ratio between the parameters in the formula shows whether the actual value of scheduled activities correlates with the planned value. A higher SPI proves that the amount of actual work is scheduled rather efficiently. If the measure tends to decline, than the work seems to be planned inefficiently and the overall schedule is likely to be inefficient as well.