There are several conventions to aid in estimating percent completion:
50/50 rule. 50% completion is assumed when the task is begun, remaining 50% - when the work is complete. It is excessively generous for the task beginning and conservative toward the end of the task.
0/100 rule. Very conservative rule. The project always seems to be running late. The earned value line will always lag the planned value line.
Critical input use rule. Task progress is defined by the amount of a critical inputs. Amount of poured concrete cubic meters -> status of the task of building the foundation.
The proportionality rule. Uses time or cost as the critical input.
The before mentioned variants can be applied to tasks not to the project itself. It the project include a lot of tasks then any of the variant applied is good enough.
The concept of earned value balances planned (baseline) cost, actual cost, schedule.
Budget at Completion (BAC).
Estimated cost at completion (EAC).
Cost (spending) variance (CV): EV - AC. Or money budgeted to accomplish the work done on current point of time minus actual cost of this work.
The schedule variance (SV) = EV - PV. Or budgeted money for the work done minus budgeted money to be spend on the work to be done on the current time.
The time variance (TV) = ST - AT. Or planned time minus actual time.
Variances can be calculated as ratios or indexes.
Cost Performance Index (CPI) = EV/AC
Schedule Performance Index (SPI) = EV/PV
Time Performance Index (TPI) = ST/AT. It is useful when an organization wishes to compare the performance of several projects. But be careful on the degree of accuracy of calculation of percent complete.
Example:
On current moment the work is expected to be costed as of 1500. The actual expenses are 1350. The completion percent = 67%.
Variances:
Cost Variance = EV - AC = 1500 * .67 - 1350 = - 350
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