Schedule performance
Earned schedule, in plain words
SPI stops telling the truth near the end of a project. Earned schedule fixes that by measuring schedule performance in time. Here is how SPI(t) works and how to read it.
The problem with SPI
The classic schedule performance index is EV divided by PV. Both are money. At the planned end date PV reaches the full budget and stops growing. EV keeps climbing until the work is done, and then it equals the budget too. So every project that finishes ends with SPI = 1.0, including the one that finished a year late.
In practice SPI becomes unreliable in the last third of a project. It improves while the project falls further behind.
The idea
Earned schedule asks a different question: at what point in the plan should we have earned the value we have today? That point is the earned schedule (ES), measured in time. Compare it with the time that has actually passed (AT) and you get schedule performance in days or months, and it stays meaningful to the last day.
| ES = C + (EV − PVC) / (PVC+1 − PVC) | C is the last whole period where planned value was at or below today's EV |
|---|---|
| SV(t) = ES − AT | schedule variance in time: negative means behind |
| SPI(t) = ES / AT | schedule performance in time |
| IEAC(t) = PD / SPI(t) | forecast duration, where PD is the planned duration |
A worked example
A ten-month project. Cumulative planned value by month, in thousands: 100, 220, 360, 520 and so on. At the end of month 4 the earned value is 400.
The plan reached 360 at month 3 and 520 at month 4, so 400 sits between them. C is 3.
| ES | 3 + (400 − 360) / (520 − 360) = 3.25 months |
|---|---|
| SV(t) | 3.25 − 4 = −0.75 months |
| SPI(t) | 3.25 / 4 = 0.81 |
| IEAC(t) | 10 / 0.8125 = 12.3 months |
The project is about three weeks behind after four months, and at this pace it finishes in 12.3 months, not 10. The money-based SPI for the same data is 400 / 520 = 0.77. Both say "behind"; only earned schedule says by how long, and only earned schedule will still say so in month eleven.
What it cannot tell you
Earned schedule is a top-down measure. It knows how much value was earned, not which activities earned it. A project can earn value on easy, non-critical work and show a healthy SPI(t) while the critical path slips. Read it next to the critical path and the forecast finish from the schedule itself. When the two forecasts disagree, find out why.
Critova shows SPI(t) beside CPI on every project and on the portfolio dashboards, with the forecast finish from the schedule next to it.
Common questions
Is earned schedule part of the PMBOK Guide?
It appears there as an extension to earned value, and it is described in PMI's practice standard for earned value management.
Do I need new data to calculate it?
No. It uses the same planned value curve and earned value you already have.
Should I stop reporting SPI?
Report SPI(t) as the schedule index. Keep SV in money if your finance team uses it.
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