Earned value divided by planned value. Below 1 means less work is complete than the baseline scheduled by that date, above 1 means more.
The schedule performance index expresses schedule progress as a ratio rather than as a sum of money, so it reads the same on a half million pound project and a half billion pound one. A project that has earned £150,000 against a planned £175,000 sits at 0.86, meaning 86 pence of work delivered for every pound the baseline expected by that date. The index measures how much work is done and never which activities are late, so it carries no view of the critical path. It also converges on 1.0 as any project completes, including one finishing a year late, which makes it a mid-flight instrument rather than a final verdict.
See also
Earned value divided by actual cost. Below 1 means the completed work cost more than its budget, above 1 means it cost less.
The budgeted cost of the work actually completed at a given date. Earned value prices finished work at baseline rates, never at what the work really cost.
Where this comes up
Two ratios off one earned value figure. SPI divides it by the plan and reads schedule. CPI divides it by the bill and reads cost. Below 1 is bad news in both, for different reasons.
Three figures read at one date. Planned value is what the baseline said would be finished by now, earned value is what did finish, actual cost is what finishing it cost.