Cost Performance Index (CPI) is a measure of the value of work accomplished as a percentage of the actual costs. In this way, CPI helps you assess spending versus budget.
The CPI is computed as CPI equals Earned Value Cost divided by Actual Cost. A CPI threshold value is expressed as a ratio. A value less than one indicates that actual costs have exceeded the value of work performed. If the lower threshold value is one, an issue is generated whenever the actual costs exceed the value of the work performed.