Schedule performance index (SPI) has three limits worth knowing. It is only as good as the progress data behind it; it measures how much work is done against plan, not how far ahead or behind you are in time, and it always ends at 1 when the work is complete; and a strong overall SPI can hide critical tasks that are behind because non-critical tasks are ahead. Read it alongside other metrics, such as earned value, schedule variance and the critical path, and lean on it most for spotting trends.
What Schedule Performance Index Measures and Where It Falls Short
What’s in a number? More than you might think. Just like miles per gallon indicates fuel efficiency, different numeric values in construction can indicate where things stand in the course of a project. Schedule performance index (SPI) is one of those numbers, telling you how you’re faring in meeting established timelines. In fact, it may be one of the more highly monitored numbers because many weighty decisions are based on what it is saying.
Yet for all the value that is placed on SPI throughout the industry, might we be relying too heavily on a simple numeric value when gauging the timely progress of an entire massive infrastructure build? After all, every metric — in our lives and our industry — can only tell us so much. It begs the question, “Is your SPI telling you the whole story?” The answer is, it depends. Here’s why.
3 Limitations of Schedule Performance Index
1. SPI Is Only as Accurate as Your Progress Data
You may have heard the saying, “you get out what you put in.” In this case, inputting true, timely data yields reliable results. And the most precise numbers are going to come straight from the job site. As progress is made on various tasks throughout the build, site crews may enter updates directly to the construction software, provided they have mobile access. Calculating your SPI using this regularly posted data should give you a good gauge of what things are on schedule and what may need immediate adjustment before worsening or impacting dependent tasks.
On the other hand, if inaccurate, incomplete, incorrect or late data is used as a basis to calculate your SPI, you’ll wind up with a skewed value. The result could give you a false sense of progress, or it could flag a false warning of a deviation. Each of these can lead to decisions based on bad data. How do you know for sure? One way is to keep an eye on other project performance metrics as well.
2. SPI Measures Work Completed, Not Time
It tells you one thing: how much work (as a percentage) has been completed thus far compared to the plan. It does not tell you exactly how far ahead or behind (as a measure of actual time). In fact, that leads to one confusing aspect surrounding this metric: at the end of a project, the value will always be “1” because 100% of the work will have been completed, regardless of whether actual completion is early or late. SPI seems to lose value as a measure of efficiency the closer you get to finishing the project.
However, schedule performance index isn’t the only benchmark contractors and project managers rely on to measure project progress. Other metrics — such as earned value (EV), schedule variance (SV) and critical path method (CPM), for example — can each play a role. Each measures a different aspect of project performance. Pair SPI with its cost twin, the cost performance index (CPI), and set an operating range for both before construction starts, so a swing means something; then check the critical path, which shows slippage in days rather than as a ratio. So if any of those values seem comparatively off the mark, the other metrics alone or combined may be able to fill in the story gaps. It’s like viewing your project from different angles for a more complete picture.
3. A Strong Overall SPI Can Hide Late Critical Tasks
That coveted SPI>1 value for your overall project looks pretty reassuring, right? Especially with task data being input from the field in real time, it may seem you’re working with accurate numbers. But you may want to see if it’s reflective of how well individual tasks are on track, especially critical ones. Why? Because if some are behind while others are ahead, it could lead to a misleading schedule performance index. There’s no way for the SPI calculation to make a distinction between tasks that are critical or dependent versus those that aren’t. This may require checking on work packages containing critical tasks to ensure they’re not truly running behind, merely masked by non-critical tasks that are ahead of schedule.
What to Check Alongside Schedule Performance Index
Here’s what each limitation can miss and what to check with it:
| Limitation | What SPI can miss | What to check alongside it |
| Input quality | Inaccurate, incomplete or late field data skews SPI, giving a false sense of progress or a false warning | Timely progress updates from the field and other performance metrics |
| Work, not time | SPI shows work completed against plan, not how far ahead or behind you are in time, and it always ends at 1 | Schedule variance and the critical path, which show slippage in days, plus cost performance index (CPI), with an operating range set for both indices |
| Critical vs. non-critical tasks | Critical tasks running behind can be masked by non-critical tasks running ahead | Work packages that contain critical tasks |
Does this mean you shouldn’t rely on your SPI as heavily as you do? Not at all. It can still serve as a solid guide in helping make those critical schedule-impacting decisions. SPI’s biggest value and relevance may be in detecting trends. And that could make a difference when working on construction projects that are more substantial or complex.
SPI and its companion metrics are most useful when they come from current field data in one place. InEight Control tracks costs, man-hours and productivity from the field and calculates live earned value metrics, including SPI and CPI. InEight Report brings those metrics into analytics, reports and dashboards, giving teams a fuller view of how much work has been accomplished at each point in the project life cycle.
