The schedule variance and the cost variance provide the amount by which the spending is behind (or ahead of) schedule and the amount by which a project is exceeding (or less than) its budget. They do not give an idea of how these amounts compare with the total budget.
The ratio of earned value to planned value gives an indication of how much of the project is completed. This ratio is the schedule performance index (SPI). The formula is SPI = EV/PV. In the John’s move example, the SPI equals 0.62 (SPI = $162.10/$261.65 = 0.62). A SPI value less than one indicates the project is behind schedule.
The ratio of the earned value to the actual cost is the cost performance index (CPI). The formula is CPI = EV/AC.
Cost Performance Index of John’s Move
In the John’s move example, CPI = $162.10/$154.50 = 1.05. A value greater than 1 indicates the project is under budget.
The cost variance of positive $7.60 and the CPI value of 1.05 tell John that he is getting more value for his money than planned for the tasks scheduled by day six. The schedule variance (SV) of negative $99.55 and the schedule performance index (SPI) of 0.62 tell him that he is behind schedule in adding value to the project.
During the project, the manager can evaluate the schedule using the schedule variance (SV) and the schedule performance index (SPI) and the budget using the cost variance (CV) and the cost performance index (CPI).
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