What is the Schedule Performance Index?

In project management, getting an early indication of problems is the silver bullet that allows the project manager to correct the problems before they start.

The Schedule Performance Index, usually abbreviated as SPI, is one of the fundamental outputs of Earned Value Management.  It tells the project manager how far ahead or behind the project is at the point of analysis (usually right now).

Formula

Where:

• SPI = Schedule Performance Index
• EV = Earned Value (dollars, euros, etc.)
• PV = Planned Value (dollars, euros, etc.)

Interpretation of Results

• If SPI is less than 1, the task is behind schedule.
• If SPI is 1, the task is on schedule
• If SPI is greater than 1, the task is ahead of schedule.

For example,

• SPI = 0 means the project work has not started.
• SPI = 0.5 means the project has performed half the work it was supposed to at this point.
• SPI = 1.0 means the project is on schedule.
• SPI = 2.0 means the project has performed twice the work it was supposed to at this point.

Background

The Schedule Performance Index represents the relative amount that the task is behind or ahead of schedule.  For example, if the task Build Fence has a budget of $4,000 and the schedule performance index is 1.25, this would represent a task that is 25% ahead of schedule.

To determine the overall project SPI, the underlying Planned Value (PV) and Earned Value (EV) must be calculated on a task by task basis and the SPI is calculated from the sum of PV and EV.

It must have a time point of reference, in other words, it is a “snapshot” at a certain point in time.  If the project is currently active, the schedule performance index is changing.  To put it another way, the project is getting more ahead or behind schedule as time goes on and as work goes on.

Schedule Baseline

In order to calculate the schedule performance index, the project must initially be divided into tasks and each task must be assigned the following data:

1. Start and Finish Dates
2. Budget

This is called the schedule baseline, and it gives the project manager something to track against.  Project scheduling is one of the fundamental aspects of project management.

Planned Value (PV)

Also known as Budgeted Cost of Work Scheduled (BCWS), Planned Value is the amount of the task that is supposed to have been completed, in terms of the task budget.  It is calculated from the project budget.

For example, if it’s Feb. 15 today, and the task is supposed to last from Feb. 10 to Feb. 20, it should be 50% complete.  If the task budget is $4,000, PV = 50% x $4,000 = $2,000.

Earned Value (EV)

Also known as Budgeted Cost of Work Performed (BCWP), Earned Value is the amount of the task that is actually completed.  It is also calculated from the project budget.

For example, if the actual percent complete is 75% and the task budget is $4,000, EV = 75% x $4,000 = $3,000.

Example

Let’s say we have a project with two tasks, building a fence and laying sod.  The initial schedule baseline is:

ID	Task	Start date	End Date	Budget
100	Build Fence	Feb. 10	Feb. 20	$4,000
200	Lay Sod	Feb. 12	Feb. 25	$3,000
TOTAL				$7,000

Let’s say it’s Feb. 15.  Determine the Schedule Performance Index for the project.

Step 1:  Determine the percent complete for each task.  Since this is not a real project we will assume the tasks are 75% and 10% complete, respectively.

ID	Task	Start date	End Date	Budget	% Complete
100	Build Fence	Feb. 10	Feb. 20	$4,000	75%
200	Lay Sod	Feb. 12	Feb. 25	$3,000	10%
TOTAL				$7,000

Step 2:  Determine Planned Value (PV).

Task 100 should be 50% complete, given the start and end dates of Feb. 10 and Feb. 20, respectively.  Therefore PV = 50% x $4,000 = $2,000.

Task 200 should be 3 out of 13 days complete, which is 23%.  Therefore, PV = 23% x $3,000 = $690.

ID	Task	Start date	End Date	Budget	% Complete	PV
100	Build Fence	Feb. 10	Feb. 20	$4,000	75%	$2,000
200	Lay Sod	Feb. 12	Feb. 25	$3,000	10%	$690
TOTAL				$7,000		$2,690

Step 3: Determine Earned Value (EV)

Task 100 is actually 75% complete, therefore EV = 75% x $4,000 = $3,000.

Task 200 is actually 10% complete, therefore EV = 10% x $3,000 = $300.

ID	Task	Start date	End Date	Budget	% Complete	PV	EV
100	Build Fence	Feb. 10	Feb. 20	$4,000	75%	$2,000	$3,000
200	Lay Sod	Feb. 12	Feb. 25	$3,000	10%	$690	$300
TOTAL				$7,000		$2,690	$3,300

Step 4: Determine Schedule Performance Index (SPI)

SPI = EV / PV.

ID	Task	Start date	End Date	Budget	% Complete	PV	EV	SPI
100	Build Fence	Feb. 10	Feb. 20	$4,000	75%	$2,000	$3,000	1.25
200	Lay Sod	Feb. 12	Feb. 25	$3,000	10%	$690	$300	0.43
TOTAL				$7,000		$2,690	$3,300

The overall project schedule performance index is $3,300 / $2,690 = 1.23, which means that 23% more work has been accomplished than the project plan at this point.  However, task 200 has an SPI of 0.43 which means that it is significantly behind schedule.

Related Earned Value Metrics

The Schedule Performance Index is usually analyzed in conjunction with the Cost Performance Index (CPI), which tells you how far over or under budget, as opposed to ahead or behind schedule, the project is.

• SPI and CPI are both greater than 1.0:  The project is ahead of schedule and under budget (hooray!)
• SPI is greater than 1.0 and CPI is less than 1.0:  The project is ahead of schedule but over budget.  In other words, more tasks have been performed than were scheduled at this point, but the tasks that have been performed are over budget.
• SPI is less than 1.0 and CPI is greater than 1.0:  The project is under budget but behind schedule.  In other words, less tasks have been performed than were scheduled at this point, but the tasks that have been performed are under budget.
• SPI and CPI are both less than 1.0:  The project is behind schedule and over budget (boo!)

The Schedule Variance (SV) is similar to SPI but tells you the absolute, rather than relative, amount the project is ahead or behind schedule.