CPI is calculated as follows:
1) If Hours left = 0, the following formula is used:
2) If Hours left > 0, the following formula is used:
SPI is calculated using the formula:
- StartDate, EndDate, Progress, HoursLeft, and ActualHours are project parameters,
- EstimatedHours is a total of estimated hours of all the project's activities plus the project's own estimated hours, and
- CurrentDate is the current date.
"Definitely bad" index
SPI is 'definitely bad' when the project's end date is in the past.
CPI is 'definitely bad' when Actual Hours > Estimated Hours and Hours left = 0.
Health color coding
The chart explains the color coding being used to show what health a given activity/project has:
|CPI/SPI Values||Color Code||Comments|
|Both indices are at or above their thresholds.||Healthy project (when one of the indices is good and the other is unavailable*, or both SPI and CPI are good)|
|One index is below and the other is above its threshold or N/A.||Activity/Project at moderate risk (CPI is good and SPI is bad, or vice versa)|
|Both indices are below their thresholds.||Project is at high risk (both CPI and SPI are below the threshold, or either SPI or CPI is definitely bad (DB))|
|* The conditions under which SPI and CPI are not calculated (not available) are given in the notes below.|
The default threshold for both CPI and SPI is 1.
The threshold is a value you set to match CPI and SPI against.
Generally, the higher the threshold the stricter your requirement for cost or schedule performance will be.
However, setting the thresholds needs to be done with care. For example, 'having a CPI that is very high (in some cases, very high is only 1.2) may mean that the plan was too conservative, and thus a very high number may in fact not be good, as the CPI is being measured against a poor baseline. Management or the customer may be upset with the planners as an overly conservative baseline ties up available funds for other purposes, and the baseline is also used for manpower planning.' (a Wikipedia article on EVA).
Calculating CPI and SPI individually
Use 'Allow calculating CPI and SPI individually' (Settings > General > Advanced settings > Project Health (Earned Value Analysis) settings) to make sure one of the indices is calculated when the other one is unavailable.
- Project's start date is preceding the current date so the project is ongoing, and
- Project's status is treat-as-open or treat-as-hold – those with a status treat-as-complete, e.g. Closed, Completed, etc. (see Workflow for details), are not included in the calculation.