Weekly/Monthly Averages

I have a worksheet which is populated with weekly data for a range of items. Each week is a seperate row and each item is a seperate column. Each row has a date in column A. The data spans several years.

I need to calculate a monthly average for each item. For example, for a single item I need to average all the "Jan-04" rows, etc.

I have tried using "If", "vlookup", "Sumif", etc. nothing seems to give me what I am looking for.

I think I'm close with SumIf (using it with a CountIf as a divisor) but I can't seem to get it right.

Any advice?

Adjust as appropriate but perhaps

=SUMPRODUCT((YEAR(A3:A13)=2006)*(MONTH(A3:A13)=1)*B3:K13)/SUMPRODUCT((YEAR(A3:A13)=2006)*(MONTH(A3:A13)=1)*ISNUMBER(B3:K13))

if the dates are in column A and the numbers go to column k

regards

Dav