Digital Engineering Guide
The Schedule Performance Index compares earned value with planned value. It is a useful indicator of schedule performance in EVM, but it should be interpreted together with the actual project schedule.
Formula
SPI = EV ÷ PV
If EV is AED 400,000 and PV is AED 500,000, SPI = 0.80.
Interpretation
- SPI = 1.00: earned value equals planned value.
- SPI < 1.00: less value has been earned than planned by the status date.
- SPI > 1.00: more value has been earned than planned.
Why SPI is not the same as time delay
An SPI of 0.80 does not mean the project is exactly 20% late in calendar time. EVM converts scheduled work into budgeted value. Critical-path logic, float and sequencing still need to be reviewed in the project schedule.
Behavior near project completion
As a project finishes, cumulative EV approaches cumulative PV and SPI tends toward 1.0, even if the project completed late. This is one reason schedule analysis should include milestone and critical-path information in addition to cumulative SPI.
Best use
Use SPI as an early-warning and trend metric, then drill into the schedule to determine which activities and dependencies are driving the variance.
Frequently asked questions
Does SPI below 1 mean the project is late?
It indicates that earned value is behind planned value at the status date, but calendar impact must be confirmed from the schedule.
Can SPI be used without a baseline?
No meaningful SPI exists without planned value derived from an approved time-phased baseline.
Should SPI replace critical path analysis?
No. It complements schedule analysis but does not replace network logic, float and critical-path review.
Reference
For formal EVM terminology see PMI Earned Value Management resources.
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