The Schedule Performance Index, or SPI, provides insight into the accuracy of the predicted project schedule and is a measure of schedule efficiency. It’s one of many key metrics used in Project Management.
The SPI formula is defined in A Guide to the Project Management Body of Knowledge (PMBOK® Guide) as follows:
“The Schedule Performance Index (SPI) is a measure of schedule efficiency, expressed as the ratio of earned value to planned value.”
The calculation displays how closely the project work is progressing compared to the initially estimated schedule.
SPI is the ratio of the performed work to the scheduled work. That is the ratio of Earned Value (EV) to Planned Value (PV).
Using the formula SPI = EV / PV, you quickly get a result that is one of the following:
- Equal to 1: the project is on schedule.
- Less than 1: the project is behind schedule.
- Greater than 1: the project is ahead of schedule.
Here’s a simple example:
Let’s suppose your scheduled project completion is in two months.
Scenario 1: After one month, 40% of the planned work is completed.
SPI = EV / PV = 40% / 50% = .8. The project is behind schedule.
Scenario 2: After one month, 50% of the planned work is completed.
SPI = EV / PV = 50% / 50% = 1.0. The project is on time.
Scenario 3: After one month, 60% of the planned work is completed.
SPI = EV / PV = 60% / 50% = 1.2. The project is ahead of schedule.
The SPI gives us insight into assessing the accuracy of the schedule as the project progresses. In addition, it can be beneficial for highlighting issues before they get worse.
Depending on what the SPI shows, we may want or need to:
- Rethink how the schedule baseline was created.
- Make mid-project corrections and revise the target completion date.
- Revise our estimation techniques going forward.
- Consider adding resources, changing requirements, redistributing workloads, or making other adjustments that would allow us to reach the initially targeted schedule.
The SPI is most effective when calculated at regular intervals throughout the project. Since falling behind schedule can often be a significant cause of project failure, tracking the SPI is a simple and proactive way to stay informed and aware of needed attention.
The frequency of calculating the SPI is also a function of the schedule duration and the type of project. The most crucial aspect is calculating it regularly and then taking action when the numbers are not indicative of successful on-time completion.
As with any measurement methodology, results need to be carefully evaluated in context.
SPI can’t distinguish whether completed tasks are critical or noncritical; it only looks at the total work achieved to date. Therefore, it would be possible to have a good SPI when noncritical tasks are ahead of schedule, even though critical tasks are behind schedule.
SPI calculations are only as good as the data collected throughout the process. The SPI calculation won’t be helpful when you haven’t correctly identified all measurable tasks or have been sloppy in tracking data.
SPI doesn’t work as a metric in isolation. SPI values must be considered part of a larger package to give a more complete and accurate story. Other factors to be considered would include Cost Performance Index (CPI) and critical path analysis.
At Thurman Co., we embrace the PMI certification principles and core metrics such as SPI as part of the foundational framework driving how we operate and interact with clients, suppliers, and partners.
We help businesses manage projects to significantly impact their success and growth. When you’re ready to put your project in the hands of a trusted professional organization, contact us to learn more about working together.