Schedule Performance Index (SPI)

The Schedule Performance Index (SPI) is the ratio of work completed (Earned Value, EV) to work planned (Planned Value, PV) at a given point in time. An SPI of 1.0 means the project is exactly on schedule — the amount of work completed matches the amount planned. An SPI below 1.0 means less work has been completed than planned — the project is behind. An SPI above 1.0 means more work has been completed than planned — the project is ahead. SPI is expressed as a pure ratio, which makes it useful for comparing performance across packages or projects of different sizes.

A critical limitation of SPI is that it converges to 1.0 at project completion regardless of actual schedule performance. If a project is severely late and finishes long after the planned completion date, EV and PV will eventually equalise because all planned work is eventually done — and the SPI will read 1.0 at closeout even though the project was months late. This means that SPI is most informative early and mid-project, and becomes progressively less useful as a forecasting tool in the later stages. Earned Schedule — a development of classical EVM that converts schedule variance from cost terms into time — was specifically developed to address this limitation.

SPI should be read alongside the Schedule Variance (SV = EV − PV) and interpreted in context. A low SPI on a non-critical work package may be irrelevant; a low SPI on a critical path activity requires immediate attention. The most common mistake is reporting SPI at programme level without disaggregating by package or work stream: an aggregate SPI of 0.95 can conceal individual packages at 0.70 being masked by others at 1.15. Always present SPI by work breakdown structure level as well as in aggregate, and always cross-reference the packages with the lowest SPI against the critical path.