Earned value divided by actual cost. Below 1 means the completed work cost more than its budget, above 1 means it cost less.
The cost performance index compares money spent against work delivered, which is a different question from money spent against money budgeted. A project that has earned £150,000 for a spend of £160,000 sits at 0.94, meaning 94 pence of budgeted work came back for every pound that went out. Dividing the budget at completion by the current index gives a rough estimate at completion, the cost of the whole project if today’s efficiency holds. The index says nothing about quality, so rework arrives as fresh actual cost with no fresh earned value behind it and pulls the ratio down.
See also
Earned value divided by planned value. Below 1 means less work is complete than the baseline scheduled by that date, above 1 means more.
The budgeted cost of the work actually completed at a given date. Earned value prices finished work at baseline rates, never at what the work really cost.
Where this comes up
Two ratios off one earned value figure. SPI divides it by the plan and reads schedule. CPI divides it by the bill and reads cost. Below 1 is bad news in both, for different reasons.
Three figures read at one date. Planned value is what the baseline said would be finished by now, earned value is what did finish, actual cost is what finishing it cost.