My excuse is that I’ve got a heavy cold, and my brain is definitely not firing on all cylinders. I’ve found a problem which ought to have a simple Excel (not VBA) solution, but I can’t work out what it is. Dick, Jon, anybody: please put me out of my misery.

I’m currently revising some documentation for a leading financial OLAP framework (ActivePivot), so I’m much involved with pivot tables. I was toying with one of my own, which summarized Sales data (in Excel, not OLAP) – the usual stuff:

(Yes, it’s old data, and I need to change the dates). Simple enough to have Min and Max of Sales:

But suppose that I want to see the *difference *between Max and Min. Surely there is somewhere to enter the formula ‘=Max – Min’ (or possibly something like ‘=Values.Max – Values.Min’). But where?

It’s not a Calculated Field, since that is essentially a calculated column on the data table. I could, for example, have a field ‘= Sales / Contacts’.

It’s not a Calculated Item, since that aggregates values for dimension members. For example, I could aggregate Jan+Feb+Mar into a Q1 value. I could even do differences, for example, Jul-Dec minus Jan-Jun. However, although the Min and Max *Values *(fields) appear as column headers, they are not a dimension.

I could have formulas that sit outside the pivot table:

- using GETPIVOTDATA on individual cells
- using an array formula to subtract the whole Min array from the whole Max array.

However, these do not, of course, adapt to changes in the pivot table layout, such as filtering member values.

Since an aggregation function could be any function that takes an array of values and returns a single value, we could have more sophisticated derived measures based on the built-in aggregation functions (Max, Average, Stdev, etc).

The OLAP framework mentioned above is of course a much more sophisticated beast. There are ways to calculate additional fields before aggregation in the hypercube (cf. pivot cache), custom aggregation functions within the hypercube, and post-processor functions to calculate additional values. I think the last of these would be the appropriate one here.

Is there a VBA solution? Right now, I haven’t penetrated deep enough into the Byzantine complexities of the pivot table object model. A quick look indicated that *PivotField.function* is an enumeration of the built-in types. So there’s no obvious hook for a custom aggregation function.

Now I’m going away to cough a lot…