Calculate Forecast Spread by Budget

A set of examples may help demonstrate how Cobra calculates forecast spread according to the current spread.

Assume that you have the following budgeted and forecast costs before statusing:

Periods	1	2	3	4	5	Total
Budget	100	200	300	200	200	1000
ETC	100	200	300	200	200	1000

After one period, assume that ETC has been calculated as follows:

ETC = BAC Earned Value = 1000 - 50 = 950

This results in the following cost data:

Periods	1	2	3	4	5	Total
Budget	100	200	300	200	200	1000
Earned Value	50					50
Actual Costs	100					100
ETC		211	317	211	211	950

Now, if the estimated finish date for the work package had been pushed back one period (to take the poor performance into account), the spread would be:

Periods	1	2	3	4	5	6	Total
Budget	100	200	300	200	200		1000
ETC		95	190	285	190	190	950

Note, however, that the profile used depends on how much has been earned. Because the earned value was less than the amount of the first period budget, the entire budget profile was used. If, however, the full amount had been earned, the profile used would have been generated from the last four periods of budget. For example, assume a performance factor of 1 and the following cost data:

Periods	1	2	3	4	5	6	Total
Budget	100	200	300	200	200		1000
Earned Value	100						100
Actual Costs	120						120

First, the spread profile curve is created. It is a four-point curve made up of the values of the last four budget amounts: 200, 300, 200, and 200, which total 900. The profile therefore is:

• Period 2 .222
• Period 3 .333
• Period 4 .222
• Period 5 .222

If expressed as a cumulative curve:

• Period 2 .222
• Period 3 .555
• Period 4 .777
• Period 5 1.0

The ETC profile is then calculated by determining each spread point:

• First spread point
  • Factor= 4 * (1/5) =.8
  • Integer= 0
  • Modulus = .8
  • Percent= 0 + (.8 * (.222 - 0)) = .178
• Second spread point
  • Factor= 4 * (2/5) = 1.6
  • Integer= 1
  • Modulus = .6
  • Percent = .222 + (.6 * (.555 - .222)) = .422
• Third spread point
  • Factor= 4 * (3/5) = 2.4
  • Integer = 2
  • Modulus= .4
  • Percent= .555 + (.4 * (.777 - .555)) = .644
• Fourth spread point
  • Factor = 4 * 4/5 = 3.2
  • Integer = 3
  • Modulus= .2
  • Percent = .777 + (.2 * (1 - .777)) = .821
• Fifth spread point
  • Factor= 4 * (5/5) = 4
  • Integer= 4
  • Modulus= 0
  • Percent= 1

These spread points are then multiplied by the ETC amount to get cumulative spread values for the ETC, and the interval values are then obtained by subtraction of the previous value:

Periods	1	2	3	4	5	6	Total
Budget	100	200	300	200	200		1000
ETC		160	220	200	160	160	900