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Intermediate · 2 min read

Schedule performance index (SPI)

Earned value’s schedule ratio — a useful trend and a dangerous single number.

Working definition

SPI is earned value divided by planned value: the ratio of work accomplished to work scheduled, as of the data date. An SPI of 0.85 says the project has delivered 85 cents of every planned dollar of progress. It compresses schedule performance into one number executives can track — which is exactly its value and exactly its danger.

Key takeaways

  • As a trend, SPI is genuinely informative: a portfolio-level view of which projects are falling behind plan and whether the gap is widening.
  • First, SPI is criticality-blind: it weighs all progress by value, so strong performance on non-critical work masks slippage on the critical path.
  • Read SPI next to the schedule facts it cannot see: critical-path movement, float consumption, and milestone variance in plain days.
01

What it honestly tells you

As a trend, SPI is genuinely informative: a portfolio-level view of which projects are falling behind plan and whether the gap is widening. It is cheap to compute wherever earned value is already measured, and it catches broad underperformance that date-watching misses — a project can hold its milestones for months while quietly under-delivering volume, and SPI shows it.

02

The two ways it lies

First, SPI is criticality-blind: it weighs all progress by value, so strong performance on non-critical work masks slippage on the critical path. A project can post SPI 1.02 while its completion date deteriorates. Second, SPI mathematically drifts to 1.0 as any project — however late — approaches completion, because planned value tops out. Late-stage SPI flatters everyone.

03

Using it without being used by it

Read SPI next to the schedule facts it cannot see: critical-path movement, float consumption, and milestone variance in plain days. "SPI 0.88 and the critical path lost twelve days this period" is a statement an executive can act on; a lone index — or worse, a composite health score built on top of one — is a number wearing the costume of an answer. Plain language beats the gauge.

Sources and scope

Follow the reference trail.

This guide is original educational commentary. Kairos reference records identify the source organizations, editions and scope behind this guidance. The original publishers retain their publications and rights.

10S-90: Cost Engineering TerminologyAACE International · checked 2026-08-01Earned Value Management definitionsU.S. Department of Defense · checked 2026-08-01DoD Earned Value Management System Interpretation GuideU.S. Department of Defense · checked 2026-08-01