The SPI or schedule performance index measures your project’s progress against planned. This one is mentioned in most schedule update meetings you will attend & also quite often in reference to contractor performance.
Schedule performance index formula
To calculate SPI, we divide our earned value by our planned value.
SPI = EV/PV
If the schedule performance index is:
- Greater than 1: We’ve done more work than planned. Our project is ahead of schedule.
- Less than 1: We’ve completed less work than planned. Our project is behind schedule.
- Equal to 1: We’ve completed the same amount of work as planned. Our project is on schedule.
Let’s use the same examples we used to calculate the schedule and cost variances.
1. We spent $12,500 in the first month and we only completed 20% of our project.
E.g.
- Project duration: 4 months
- BAC: $40,000
- Elapsed time: 1 month
- Planned value: $10,000
- Actual cost: $12,500
- (planned) % complete: 25%
- (actual) % complete: 20%
- Earned value = $8,000
SPI = EV/PV
SPI = $8,000/$10,000
SPI = Actual 20% / Planned 25%
SPI = 0.8 (behind schedule)
This means that for every estimated hour of work the project team is only achieving 0.8h of work which is 48min (60min x 0.8 = 48min) So because our SPI is less than 1, our project is behind schedule.
This same calculation can be applied to work hours planned vs work hours executed to produce a performance value to forecast project completion dates.
2. We spent $27,500 after three months. All our activities scheduled up to our data date have been completed.
E.g.
- Project duration: 4 months
- BAC: $40,000
- Elapsed time: 3 months
- Planned value: $30,000
- Actual cost: $27,500
- (planned) % complete: 75%
- (actual) % complete: 75%
- Earned value: $30,000
SPI = EV/PV
SV = $30,000 / $30,000
SV = Actual 75% / Planned 75%
SV = 1 (on schedule)
You can learn more about Earned Value Management here.
