The Cost Performance Index (CPI) is the ratio of Earned Value to Actual Cost and is used to estimate the projected cost of completing the project.
CPI = EV / AC
A CPI greater than 1 means that the cost of completing the work is less than planned (under budget).
A CPI less than 1 means that the cost of completing the work is higher than planned (bad).
A CPI equals to 1 means that the cost of completing the work is right on plan (good).