Average of averages Excel formula

Question

how do I make a single formula that will take the n-week average of m-week averages?

Suppose I have data in A2:A21, and in D21, I want the 8 week average of 4-week moving averages up to A21. So my idea would be

{=AVERAGE(AVERAGE(OFFSET(A21,-4+1-ROW(OFFSET(A1,0,0,8))+1,0,4)))}

So for example, with this data:

data    4 wk aver   8 wk avg avg
79      
79      
66      
38  66  
34  54  
13  38  
22  27  
50  30  
40  31  
98  53  
81  67  46
27  62  45
64  68  47
12  46  48
83  47  50
94  63  54
4   48  57
41  56  57
4   36  53
6   14  47

I am looking for a formula to calculate 47... My formula above calculates 57.8.

Answer 1

Answer is to use SUBTOTAL() on the inner average instead. This will return a column of averages, so the outer AVERAGE() will have the correct behavior.

{=AVERAGE(SUBTOTAL(1,OFFSET(A21,-4+1-ROW(OFFSET(A1,0,0,8))+1,0,4)))}

The reason the original solution didn't work is because the inner AVERAGE was just taking the total average of all the arrays, instead of returning a column of their individual averages. I find this SUBTOTAL() trick useful when you need to do this kind of thing.