excel - Pivot Calculated formula: SUM(Field1)/AVG(Field2)

Question

I've a simple table with some amount and interval in sec by date and product name.

Month | Product | Amount | Interval in sec
------------------------------------------
05-'12| Prod A  | 10     | 5
05-'12| Prod A  | 3      | 5
05-'12| Prod B  | 4      | 5
05-'12| Prod C  | 13     | 5
05-'12| Prod C  | 5      | 5

From this table I've derived a PivotTable with SUM(Amount), AVERAGE(Interval in sec) by Month and Product.

Month | Product | SUM of Amount | AVG of Interval in sec
--------------------------------------------------------
05-'12| Prod A  | 13            | 5
05-'12| Prod B  | 4             | 5
05-'12| Prod C  | 18            | 5

So far so good. Now I want to add and extra column to my PivotTable with gives me the outcome of SUM of Amount / AVG of Interval in sec.

Adding a calculated value =SUM(Amount)/AVERAGE(Interval) is not giving me the right values. Excel gives me:

Month | Product | SUM of Amount | AVG of Interval in sec | Amount per sec
-------------------------------------------------------------------------
05-'12| Prod A  | 13            | 5                      | 1.3
05-'12| Prod B  | 4             | 5                      | 0.8
05-'12| Prod C  | 18            | 5                      | 1.8

What it actually is doing is =SUM(Amount)/SUM(Interval in sec) for every Month and Product based on the values in the first table. But I'm looking for:

Month | Product | SUM of Amount | AVG of Interval in sec | Amount per sec
-------------------------------------------------------------------------
05-'12| Prod A  | 13            | 5                      | 2.6
05-'12| Prod B  | 4             | 5                      | 0.8
05-'12| Prod C  | 18            | 5                      | 3.6

So literally divide pivot field 'Sum of Amount' by pivot field 'AVG of Interval in sec'.

How to achieve this?

Answer 1

In case anyone else comes across this problem, a solution that I've found is to add a helper "Count" column to your data set, where each record has a 1 entered under the "Count" field.

Then, in order to achieve the effect of an Average, you can use:

SUM(FIELD_TO_AVERAGE)/SUM(COUNT)

I believe this should work in all cases where an average is needed, but I haven't tested it very broadly.