Examples of Statistical Forecast Calculations

This example demonstrates how statistical forecast calculations are performed in Cobra.

For the following examples of the different statistical forecast calculations, assume that forecasts are being generated following the closing of period 8 for a work package with the following budgeted, earned value, and actual costs:

	Period 1	Period 2	Period 3	Period 4	Period 5	Period 6	Period 7	Period 8	Period 9	Period 10	Total
Budget	100	100	100	100	100	100	100	100	100	100	1000
Earned Value	100	75	75	75	100	75	100	100			700
Actual Costs	100	100	100	100	125	100	125	150			900

PF = 1

This method always calculates ETC by subtracting Earned Value from BAC, which implies that performance is following the budgeted schedule (that is, assumes a performance factor of 1):

ETC = 1000 – 700 = 300

EAC = 900 + 300 = 1200

PF = 1/CPI Cum to date

Uses the cumulative-to-date CPI, which trends ETC based on the total past performance of work completed.

PF = 900/700 = 1.2857

ETC = 1.2857 * 300 = 386

EAC = 900 + 386 = 1286

PF = 1/CPI current period

This method calculates a performance factor using a CPI based on the most recent fiscal period (in the following example, period 8):

PF = 150/100 = 1.5

ETC = 1.5 * (1000-700) = 450

EAC = 900 + 450 = 1350

PF = 1/CPI last 3 periods

This method calculates a performance factor using a CPI based on the three most recent fiscal periods (in the following example, periods 6, 7, 8):

PF = 375/275 = 1.3636

ETC = 1.3636 * 300 = 409

EAC = 900 + 409 = 1309

PF = 1/CPI last 6 periods

This method calculates a performance factor using a CPI based on the six most recent fiscal periods (in the following example, periods 3, 4, 5, 6, 7, 8):

PF = 700/525 = 1.3333

ETC = 1.3333 * 300 = 400

EAC = 900 + 400 = 1300

PF = User input

This method allows the user to input a PF value. In this example, assume that the user entered a value of 1.1:

PF = 1.1

ETC = 1.1 * 300 = 330

EAC = 900 + 330 = 1230

PF = 1/(a*CPI) + (b*SPI)

For this method, the calculation of the performance factor is based on cumulative-to-date SPI and CPI, allowing the user to define what proportion of each should be used, where a and b are the proportional factors and a + b = 1. In this case, assume the user has set both a and b to 0.5:

PF = 1/((.5 * 700/900) + (.5 * 700/800)) = 1.21

ETC = 1.21 * 300 = 363

EAC = 900 + 363 = 1263

Defining a as zero and b as 1 results in the following calculations:

PF = 1/(0 + (700/800)) = 1.143

ETC = 1.143 * 300 = 343 (342.86)

EAC = 900 + 343 = 1243

PF = 1/(CPI * SPI)

This method provides a worst-case scenario by multiplying the cumulative-to-date performance indices together, thereby heightening the effect of cost and schedule variance.

PF = 1/((700/900) * (700/900)) = 1.653

ETC = 1.653 * 300 = 496

EAC = 900 + 496 = 1396

Percent Complete Ranges

This method allows for changing the calculation of the performance automatically depending upon the percent complete of the level at which the performance is being calculated. Cobra determines this percentage using the following formula:

Percent complete = 100 * (Earned Value/BAC)

For this example, assume the following ranges and methods have been defined:

Range	Method
0 – 25	PF=1
26 – 80	PF=1/CPI
81 – 100	Method A (Retain EAC, a manual forecasting method)

The current percent complete of the work package is calculated as follows:

Percent complete = 100 * (700/1000) = 70

As a result, Cobra uses method 2 and calculates the EAC as 1286.