Just coming back onto some cube work for the first time in 12 months or so, and managed to get myself confused by the AverageOfChildren semi-additive aggregation type, just like I did the first time round :-/
(Aside: AverageOfChildren is a semi-additive average: it sums over all dimensions except time, and averages over time. This makes sense (for example) if you want to compare average monthly sales over the year for each of your branch offices. It’s useless, however, when you want to know the average sale price per unit)
Whilst creating calculated measures of the type [Measures].[Something Sum] / [Measures].[Something Count] is all very well, I don’t understand why this isn’t just supported out of the box, as a native Average aggregation type. Sure there are always ‘flavours’ of averages that need specific MDX (weighted averages for example), but surely the simple flat average over row count is so common as to warrant ‘out of the box’ support.
You might say having two ‘average’ aggregations would confuse, but AverageOfChildren causes plenty of confusion on it’s own: having two (and documentation on which to use when) would probably make the situation considerably better.
Fool me twice: shame on me.
Update June 2011: Rather than just complain, I raised a Connect Issue: Add AverageOfRows aggregation type for simple averages. Please vote for it