Digital Engineering Guide
The Cost Performance Index measures cost efficiency in Earned Value Management. It compares the budgeted value of completed work with the actual cost incurred to achieve that work.
Formula
CPI = EV ÷ AC
If EV is AED 400,000 and AC is AED 500,000, CPI = 0.80.
How to interpret CPI
- CPI = 1.00: earned value and actual cost are equal.
- CPI < 1.00: the project is earning less budgeted value than the cost being spent.
- CPI > 1.00: the project is earning more budgeted value than the cost being spent.
What CPI does not tell you
CPI alone does not tell you whether the project is on schedule, whether remaining work is riskier, or whether costs have been properly accrued. A project can have a good CPI and still be badly behind schedule.
Using CPI for forecasting
Some EAC forecasting methods assume future cost efficiency will resemble past cost efficiency. That assumption must be tested against project reality. One-time procurement events, mobilization costs or a major change in remaining scope can make historical CPI a poor predictor.
ERP integration
Automated CPI calculation is only as reliable as the data feeding it. Actual cost should flow from controlled finance/procurement transactions and EV should come from approved progress measurement rather than arbitrary invoice percentages.
Frequently asked questions
Is a CPI above 1 always good?
It indicates favorable cost efficiency for the measured work, but the result still needs context, data-quality checks and schedule review.
Can CPI be negative?
EV and AC are normally non-negative, so CPI is generally non-negative. Data anomalies can still produce meaningless results.
Should management use one project-wide CPI?
A top-level CPI is useful, but problem areas can be hidden by aggregation. Work-package and control-account views are often more actionable.
Reference
See PMI resources on Earned Value Management.
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