Difference between Cost Performance Index (CPI) and Schedule Performance Index (SPI)

In project management, evaluating performance is critical during both development and post-development phases. Two key metrics from Earned Value Management (EVM) are the Cost Performance Index (CPI) and Schedule Performance Index (SPI). CPI measures cost efficiency, while SPI measures schedule efficiency.

Cost Performance Index (CPI)

CPI represents the amount of work being completed for every unit of cost spent. It measures how efficiently the project budget is being utilized.

CPI = Earned Value (EV) / Actual Cost (AC)

Schedule Performance Index (SPI)

SPI represents how close the actual work completed is compared to the planned schedule. It measures whether the project is ahead of, on, or behind schedule.

SPI = Earned Value (EV) / Planned Value (PV)

Interpreting the Index Values

Example

A project has the following values −

Planned Value (PV)  = $100,000
Earned Value (EV)   = $80,000
Actual Cost (AC)    = $90,000

CPI = EV / AC = 80,000 / 90,000 = 0.89
    ? Over budget (spending more than earning)

SPI = EV / PV = 80,000 / 100,000 = 0.80
    ? Behind schedule (completed less than planned)

Both values are below 1, indicating this project is over budget and behind schedule.

Key Differences

Conclusion

CPI measures how efficiently budget is being used (EV/AC), while SPI measures how well the project follows the planned timeline (EV/PV). Both indices use Earned Value as the numerator, and a value above 1 indicates favorable performance in their respective dimension.