# Software Engineering | Schedule Performance Index (SPI)

Schedule Performance Index (SPI) describes how efficiently we are actually progressing compared to the planned project schedule. The Schedule Performance Index (SPI) can be defined as a measure of schedule efficiency expressed as the ratio of earned value to planned value.

The Schedule Performance Index gives the information about the schedule performance of the project. It is the efficiency of the time utilized on the project.

Calculation of Schedule Performance Index (CPI):
The Schedule Performance Index can be computed by dividing earned value by planned value.

`SPI = EV / PV `

Where,

```Earned Value (EV): Earned Value is the amount
of the task that is actually completed.

Planned Value (PV) = Planned Completion * Task Budget ```

Interpretation of SPI:

1. SPI > 1:
If the SPI is greater than one then more work has been completed than the planned work. In other words we can say that project is ahead of schedule.
2. SPI < 1:
If the SPI is less than one then less work has been completed than the planned work. In other words we can say that project is behind schedule.
3. SPI = 1:
If the SPI is equal to one then work is being completed at about the same rate as planned. In other words we can say that project is on time.

Example:
Given a project to be completed in 12 months and the budget of the project is 1, 00, 000 INR. Six months have passed and 60, 000 INR has been spent but on closer review it is found that only 40% of the work has been completed till now.
Find the Schedule Performance Index and conclude whether the project is behind or ahead of schedule.

Solution:

Actual Cost (AC) = 60, 000 INR

Planned Value (PV) = 50% of 1, 00, 000 INR = 50, 000 INR

Earned Value (EV) = 40% of 1, 00, 000 INR = 40, 000 INR

SPI = Earned Value / Planned Value = 40, 000 / 50, 000 = 0.8

Hence, the Schedule Performance Index is 0.8.

Conclusion is that the project is behind schedule since the Schedule Performance Index is less than one.

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