What is Schedule Performance Index (SPI)?
SPI measures schedule efficiency: the rate at which work is actually being completed versus the rate the plan called for. SPI of 1.0 means exactly on schedule; below 1.0, behind; above 1.0, ahead.
One exam-worthy caveat: near project completion, SPI drifts back toward 1.0 even on late projects (because EV eventually catches PV). For late-stage schedule truth, look at the critical path, not SPI.
Formula
SPI = EV ÷ PV
Worked example
EV = $35,000, PV = $40,000. SPI = 35,000 ÷ 40,000 = 0.875 — the project is progressing at about 87.5% of the planned rate. Below 1 is behind schedule; above 1 is ahead.