According to the PMBOKGuide, specifically in the Control Costs process within the Project Cost Management knowledge area, the Cost Performance Index (CPI) is the specific metric used to measure the cost efficiency of a project.
* Definition of CPI: CPI is a measure of the cost efficiency of budgeted resources, expressed as the ratio of earned value ($EV$) to actual cost ($AC$). The formula is:
$$CPI = \frac{EV}{AC}$$
* Efficiency Indicator: Because it is an index (a ratio), it tells you how much value you are getting for every dollar spent.
* A CPI of 1.0 indicates the project is exactly on budget (spending $1 to get $1 of work).
* A CPI greater than 1.0 indicates that the work is being performed with better efficiency than planned (under budget).
* A CPI less than 1.0 indicates that the work is being performed inefficiently (over budget).
* Importance: CPI is considered the most critical EVM metric as it influences the calculation of the Estimate at Completion (EAC). It provides a clear snapshot of how efficiently the project team is using the financial resources allocated to the project.
Why other options are incorrect:
* Option A: Cost variance (CV): While CV also relates to cost performance, it is expressed as a currency value ($CV = EV - AC$) rather than a ratio. It shows the magnitude of the deviation from the budget, but not the " efficiency rate " or " percentage " of efficiency.
* Option C: To-complete performance index (TCPI): TCPI is a measure of the cost performance that must be achieved with the remaining resources to meet a specific goal (like the original BAC or a new EAC). It describes the efficiency required for the future, not the efficiency of the work already completed.
* Option D: Variance at completion (VAC): VAC is a projection of the final budget deficit or surplus ($VAC = BAC - EAC$). It is a forecasting metric used to see where the project will end up, not a measure of current work efficiency.