The Schedule Performance Index (SPI) is an Earned Value Management (EVM) schedule performance factor metric primarily used as an element of the EVM Estimate at Completion (EAC) equation. It measures the work accomplishment efficiency, by dividing the Budgeted Cost for Work Performed (BCWP) by the Budgeted Cost for Work Scheduled (BCWS).

The SPI metric is more valuable at the beginning of the contract. Unfavorable SPIs are a good forecast of future unfavorable cost metrics and indicate the potential for a schedule slip. SPIs greater than 1.0 are favorable, SPIs less than 1.0 are unfavorable. If observed, negative SPIs should be questioned.

By definition, the SPI at the end of the contract is 1.0 because when all of the planned work is completed, BCWP equals BCWS. The SPI is independent of the critical path schedule. So an unfavorable SPI metric does NOT necessarily mean a schedule slip. A schedule slip can only be determined by evaluating the network schedule critical path.

Given an SPI of 1.1, the plain language definition is: for every dollar of work scheduled for completion a dollar and ten cents worth of work was completed; likewise an SPI of 0.95 would be: for every dollar of work scheduled for completion, only 95 cents worth of work was actually completed.