average of a sample

[steveniow]

I have a sample of data calculated by taking a sum every day over a date range. I need an average of this and using avg(TOTAL aggr(sum(NoEpisodeError),LoadDate)) have done so with no problem.

However what I need to do now is get an average in the same table but only over a certain subset of those dates., and I cannot work out how to do it. 

progress so far is below. can anyone make any suggestions please?