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Before we can begin we need to be clear about what mean by forecast variance.  Given a forecast of F and an actual call volume offered of A, the two most common definitions are:

a)     (A – F) / F and

b)    (A – F) / A

Consider the following figures:

 

Week 1

Week 2

Week 3

Forecast

10,000

10,000

10,000

Actual

10,000

8,000

12,000

(A – F) / F

0%

20%

20%

(A – F) / A

0%

-25%

-17%

 

In many cases, it is preferred to express the forecast variation in relation to the calls answered.  As the table above shows, expressing the variation in relation to the forecast means that a positive or negative variation leads to the same absolute variation in percentage terms, and many people simply find this easier to work with (consider the table if the actual volume had been 5,000 or 15,000.  (A – F) / F is the definition we’ll use.