Once the CPI/SPI were calculated at some time-stamp, project manager should be deeper analyses, he need to predict the future cost:
ETC (Estimate To Completion)
Then EAC (Estimate At Completion) = AC + ETC
How to calculate ETC based on current situation (EV/AC/PV and SPI&CPI), there are usually three methods:
Regardless of current Schedule/Cost Performance Index
This way is simple, I don’t care what’s happened, EAC = BAC + EAC – AC, in other words, project team is confident to finish the project below estimated cost.
Estimated based on current CPI
This way is straight-forward, review current CPI and estimate future cost based on it:
EAC = AC + (BAC -EV)/CPI = BAC / CPI
Consider both CPI/SPI impact
I think this way the rationalist one, it consider both SPI and CPI, in additional, PM could give a “weight” to SPI or CPI according to his/her experience and judgement, so the formula is:
EAC = (BAC – EV) / (CPI * SPI)
No matter which method PM chooses, as long as the calculated/predicted EAC is not acceptable, then it is an “early warning” to the team.
Even more, PM need calculate TCPI, the content below as copied from EVM on Wikipedia, it is very clear for me.
The To Complete Performance Index (TCPI) provides a projection of the anticipated performance required to achieve either the BAC or the EAC.
For the TCPI based on BAC (describing the performance required to meet the original BAC budgeted total):
or for the TCPI based on EAC (describing the performance required to meet a new, revised budget total EAC):
Independent estimate at completion (IEAC)
The IEAC is a metric to project total cost using the performance to date to project overall performance. This can be compared to the EAC, which is the manager’s projection.
Wish I success, again! +U!