Cost Performance Index (CPI)
A ratio that compares the value of work actually completed to the money actually spent getting it done, used to forecast whether a project is on track to finish on budget, over budget, or under budget. Estimate at Completion (EAC), the budget divided by the CPI, turns that ratio into a forecasted final cost.
Why it matters
A crew can be under budget purely because it hasn't done much work yet, or over budget while still being highly efficient for the amount of work completed. CPI separates spending from productivity, measuring whether each dollar spent is actually buying a dollar's worth of completed work, which a simple budget-versus-actual comparison can't show on its own.
On a real project
A project has completed sixty percent of its budgeted scope but has spent sixty-five percent of its budget doing so, giving it a CPI below one. The team uses that ratio to calculate an Estimate at Completion that's higher than the original budget, flagging a cost problem well before the project actually runs out of money.
Who this matters most to
A Cost Engineer calculates CPI regularly throughout a project to catch a cost overrun trend early, while there's still time to correct it.
Where this goes wrong
A project tracks spending against budget but never calculates CPI, so a crew that's burning money faster than it's completing work looks fine on a simple spending report, since the project hasn't spent its whole budget yet. By the time anyone notices the real trend, there's no longer enough budget left to fix it.