Why is schedule variance measured in dollars, not days?

Because earned value and planned value are both dollar figures, their difference is in dollars too, even though it is a schedule reading. Schedule variance of negative six hundred thousand means six hundred thousand dollars of planned work has not been done yet, not that the project is some number of days late. This is the main limitation of classic earned value: it tells you the size of the lag in budget terms but not in time. Converting that into weeks or days is exactly what earned schedule was invented to do.

What does an SPI of 0.90 mean?

It means the project is completing work at ninety percent of the planned pace. For every planned dollar of work that should have been earned by now, only ninety cents has been. On our example project, planned value is six million but earned value is five point four million, so SPI is zero point nine and the project is running behind. An SPI of 1.0 means the work is keeping exactly to the planned pace, and above 1.0 means it is running ahead of plan.

Why does SPI drift toward 1.0 at the end of a project?

Because by definition all the planned value is eventually earned. At completion, earned value equals planned value equals the full budget, so SPI mechanically returns to 1.0 even on a project that finished months late. This is the well-known blind spot of the schedule performance index: near the end it stops reflecting reality. Earned schedule fixes it by measuring the lag along the time axis instead of the dollar axis, so a late finish still reads as late right up to the last day.

Construction Glossary · Earned Value

Schedule Variance &SPI

The schedule half of earned value, and the one with a catch: it reads the lag in dollars, not days, and it quietly heals itself at the finish line.

Definition

The schedule performance index (SPI) is earned value divided by planned value; it measures how much of the planned pace the work is actually keeping. Schedule variance (SV) is earned value minus planned value, the same reading in dollars. An SPI below 1.0, or a negative SV, means the project is behind schedule.

Earned pace vs planned pace

Both readings compare the work done against the work the plan called for by now. On our $12.0M project, the plan said $6.0M would be earned by month 8, but only $5.4M is.

SV = EV − PV = $5.4M − $6.0M = −$0.6M
SPI = EV ÷ PV = $5.4M ÷ $6.0M = 0.90

Schedule variance gives the lag in money; SPI gives it as a pace you can compare across projects. Both say the same thing here: the work is running behind plan.

The two catches to know

It reads in dollars, not days. A schedule variance of −$0.6M tells you how much planned work is outstanding, not how many days late you are. Turning that into weeks takes a different method.

It heals itself at the end. Because all planned value is eventually earned, SPI returns to exactly 1.0 at completion even on a project that finished late. Near the finish, the index stops telling the truth.

Both problems are why earned schedule exists: it measures the lag along the time axis, so a late job reads late right up to the last day.

How POD reads schedule performance

POD derives SPI and schedule variance from the earned value and the baseline planned value it already holds, and pairs them with its schedule reading so the dollar lag and the critical-path picture sit side by side. It reads float and the critical path from imported P6 and MS Project files rather than recomputing them, so the schedule story stays anchored to the contractor's own program.

Earned schedule & SPI guide

Frequently asked questions

Cost and schedule, read together

SPI pairs with the cost index to place a project in the earned-value quadrant, then both feed the forecast to completion.

Cost Variance & CPI Back to the Glossary