Schedule Performance Index (SPI)
Telling You the Whole Story?
May 06, 2021
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.
An accurate schedule performance index relies on accurate input
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.
SPI is only one part of the larger story of your project
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. 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.
A great SPI may not actually be as great as it seems
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.
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.
InEight Report is a software tool you can use to provide a more complete 360° view of how much work you’ve accomplished at different points in your project life cycle, including your SPI and related values. Gaining visibility and creating immediate reports using real-time data can help keep all phases of your large-scale projects on track. Want to see how it works? We’re happy to show you a demo of how it can tell your project story.