A Beginner’s Guide to the Schedule Performance Index

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The Schedule Performance Index (SPI) measures how closely your project follows the schedule. In this guide, we share some examples and what to look out for when working with SPI.

The project schedule is an important project tool. It communicates many things: what tasks need to be done and when, the resources allocated to complete tasks, due dates, the milestones the team has achieved and has yet to achieve, and so on.

A project that falls behind schedule can cause a number of events that affect project success:

For this reason, no matter what project management framework they use, project managers use a variety of techniques and tools—Gantt charts, PERT charts, the prioritization matrix, etc.—to keep things on schedule.

And then there’s this indicator called the Schedule Performance Index (SPI) that measures the actual performance of the project versus the schedule.

Overview: What is the Schedule Performance Index (SPI)?

The Schedule Performance Index is a subset of Earned Value Management (EVM), a popular but controversial way of measuring project performance. EVM shows how the work done to date measures the baselines set during the project planning phase.

Both plan variance (SV), also an EVM calculation, and SPI measure whether a project is behind, on, or ahead of schedule. SV measures how much actual work varies from planned schedule, while SPI is the ratio of work done to planned work.

It quantifies the planning efficiency and shows the status of the project compared to the schedule in percentage terms.

You can find more information about the timetable deviation calculation and examples here in our SV article.

Schedule Performance Index (SPI) vs Cost Performance Index (CPI): What is the Difference?

The Cost Performance Index (CPI) is also an earned value metric. While SPI measures planning efficiency, CPI measures project cost-effectiveness. It is the ratio of work completed to date to the total amount spent to complete the work.

The CPI formula is:

  • Cost Performance Index (CPI) = Earned Value (EV) / Actual Cost (AC)
  • CPI = EV/AC

If the CPI calculation:

  • same 1: The project is on budget.
  • Less than 1: The project is over budget. The value of the work done is less than the money spent.
  • Greater than 1: The project is under budget. The value of the work done is greater than the money spent.

To calculate earned value:

  • Earned Value (EV) = % of work completed x Budget at Completion (BAC)
  • EV = % Actual Complete x BAC

Budget at completion is the total project budget.

As for the AC, you get all costs related to the work done, including labor, machinery, travel, software licenses, etc.

Cost Plan Index (CSI)

Another earned value formula worth mentioning is the cost plan index. It measures overall project effectiveness and indicates the likelihood of a project that deviates from baseline recovering.

It is calculated as:

  • Cost Plan Index (CSI) = Cost Performance Index (CPI) x Schedule Performance Index (SPI)
  • CSI = CPI x SPI

The further the CSI is from 1.0, the less likely it is that a project that is late and/or over budget will recover.

What is the formula for the Plan Performance Index?

To illustrate the relationship between Earned Value and Planned Value required to calculate the SPI, take a look at this chart:

A line chart showing that the earned value is greater than the projected value to date.

Here we see that the work done so far (earned value) is greater than the planned work (planned value). Image source: author

To calculate the SPI performance of your project, the formula is:

  • Schedule Performance Index (SPI) = Earned Value (EV) / Planned Value (PV)
  • SPI = EV/PV

If the SPI calculation gives a value that is:

  • Greater than 1: The project is ahead of schedule.
  • Less than 1: The project is behind schedule.
  • same 1: The project is on schedule.

We already know how to calculate earned value. To find planned value, multiply the percentage of planned work by the project’s budget.

  • Planned Value (PV) = Planned percentage of completion x Budget at Completion (BAC)
  • PV = % Planned Completion x BAC

So, for example, if your project is scheduled for two weeks and it’s now exactly a week since you started, your project should already be 50% complete according to the schedule.

Let’s look at some examples.

Example SPI calculation #1

Project #1 has a budget of $5,000 and is scheduled to last 30 days. To date, 50% of the work has been completed, which is in line with the project schedule.

Table with six columns and three rows showing the project's budget and schedule.

50% of the work is completed as planned, but the actual cost is $500 more than planned. Image source: author

Based on the numbers on the sheet, the earned value is:

  • EV = % Actual Complete x BAC
  • EV = 50% x $5,000
  • EV = $2,500

The previous planned value is:

  • PV = % Planned Completion x BAC
  • PV = 50% x $5,000
  • PV = $2,500

Substituting EV and PV into the SPI formula, we get:

  • SPI = EV/PV
  • SPI = $2,500 / $2,500
  • SPI = 1.00

Because SPI equals 1, the project is on schedule. For every hour you put into the project, you get an hour of work.

Well, if you think, sure, it’s on schedule but it’s over budget, then you’re right. But SPI is not designed to measure cost effectiveness. That is the job of the CPI – hence the “cost” in the cost performance index.

Table showing the project's achieved and planned values, as well as schedule and cost performance indices.

This project is on schedule but over budget as reported by the CPI. Image source: author

How to calculate the CPI:

  • CPI = EV/AC
  • CPI = $2,500 / $3,000
  • CPI = 0.83

Because the CPI is less than 1, the project is over budget. For every $1 you spend on the project, you’ll get $0.83 back.

Example SPI calculation #2

Next we have a 15 day project with a budget of $10,000. Three quarters of the project is scheduled to be completed today, but the project team exceeded expectations by completing 80% of the work instead of 75%.

A table showing a project's budget and schedule and the project's performance.

The project is early. The work completed so far is larger than planned. Image source: author

The earned value of the project is:

  • EV = % Actual Complete x BAC
  • EV = 80% x $10,000
  • EV = $8,000

For plan value:

  • PV = % Planned Complete x BAC
  • PV = 75% x $10,000
  • Present value = $7,500

If we now calculate for SPI, we have:

  • SPI = EV/PV
  • SPI = $8,000 / $7,500
  • SPI = 1.06

Because SPI is greater than 1, our project is ahead of schedule. For every hour invested, you get back 1.06 hours of work.

But what about the CPI? How to get the CPI:

  • CPI = EV/AC
  • CPI = $8,000 / $8,000
  • CPI = 1.00

In terms of cost efficiency, the project is on budget. You get a dollar’s worth of work for every dollar you spend on it.

A section of a spreadsheet detailing a project's budget, schedule, and work completed to date.

Based on our SPI and CPI calculations, the project is on budget and ahead of schedule. Image source: author

3 tips for working with the timetable performance index

Although SPI is useful for measuring project performance, it has limitations. When working with SPI, be sure to keep the following considerations in mind.

Tip 1: A good SPI is not always good

SPI cannot distinguish critical from non-critical tasks—that is, non-critical tasks that are ahead of schedule may hide critical tasks behind schedule. Therefore, SPI can indicate that a project is moving at a good pace, when in reality it is not.

Also, the SPI is always marked 1 at the end of each project, regardless of the actual completion date. Even if you deliver the project six months past the due date, SPI says it’s on schedule.

Tip 2: SPI is only as good as the data you keep

SPI is highly dependent on the accuracy of the data you feed it. If the data you are working with is problematic, e.g. B. incomplete work stoppages or late cost reports, SPI will not help much. Garbage in, garbage out.

Tip 3: SPI should not be used alone

To get an accurate view of the project’s schedule performance, SPI scores are best analyzed alongside other project management techniques, most notably the critical path method (CPM). That way, if SPI indicates one thing and CPM indicates another, dig deeper and uncover the real story.

Keep your project plan on track

SPI is just one of several ways to stay in control of the project schedule. While it’s not perfect, it’s undoubtedly a useful tool to ensure the project is following its established roadmap.

To help your project team collaborate on projects more efficiently and keep track of your project’s schedule better, consider some of the project management software options that The Ascent reviewed. We rate each based on ease of use, features, pricing, and quality of support.

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