Ok, I freely admit that many of the articles that I post are selfishly for myself, so that in 6 months time when I have a 'now I did that 6 months ago, how did I do it' moment, I can refer back here. This is one of those.
If you need averages based on sub-grouping record counts in your reports then you need to know about The Power of One.
Many thanks to Brent Downey at adminhero.com.