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How Cobra Spreads ETC

When calculating forecast costs, Cobra spreads the ETC for each work package or control account between the estimated start and finish dates (or between the status date and the estimated finish date for in-progress work).

In addition, Cobra lets you explicitly update the spread of ETC in a manner similar to the spreading of the budget in control account planning.

To be accurate, forecast calculations must take into account that schedule or rate variances (either positive or negative) may result in the use of rates different from the ones used to budget the work. Thus, Estimate to Complete (ETC) calculations must take into account how future expenditures will be spread over time.

Cobra attempts to retain the current spread profile when the new forecasts are spread. The profile used depends on both the existence of a current ETC spread and upon the setting of the project option that allows users to use the current spread for forecast spreads. If there is no current ETC spread or if the project option is set to spread according to the current spread, the profile is generated from the current spread profile. The included current spreads for the forecast currently being spread are totaled for the resource by date, and the period current spread values are totaled and normalized into percentages, which make up the curve point values. The profile curve has as many points as there are periods in the current spread detail, and Cobra stretches or shrinks the curve as necessary if the dates for the work are adjusted at a later date.

Cobra contains special algorithms that preserve the previous spreading of ETC, if at all possible:

  • If only the performance factor has changed, the previous spread is retained and the new ETC is prorated against it.
  • If the dates have changed but the number of fiscal periods has not changed, the previous profile is simply shifted to match the new dates and is re-rated.
  • If the new dates span a different number of fiscal periods than the current spread, Cobra attempts to reproduce the existing curve within the new dates. This may result in a stretching or shrinking of the curve, based on whether the number of fiscal periods increases or decreases.

The spread is done by calculating the percentage of the ETC that will go into each of the periods. This is done by mapping the number of periods to be spread into over the number of periods currently making up the profile, weighted by the profile values that are normalized as cumulative percentages (total to 100). Then the number of the period to spread is divided by the total number of periods to be spread into and multiplied by the number of profile points to get a factor. The factor is made up of an integer value and a modulus. The integer value represents the profile point after which the spread point will fall. The modulus is the percentage of the difference between the new two profile points.

To better understand how Cobra spreads ETC, see the Calculate Forecast Spread by Existing EAC and Calculate Forecast Spread by Budget examples.