I have 3 cols such that they look like this:

colA colB colC colD
1931jfm 1931jan 1 mean of 1930(j,a,s,o,n,d)
1931amj 1931feb 2 mean of 1930(o,n,d),1931(j,f,m)
1931jas 1931mar 3 ....
1931ond 1931apr 4 .....

where j is jan, f is feb, m is march and so on
1930(j,a,s,o,n,d) is jul1930, aug1930,sep1930 and so on till dec1930
I want to calculate the mean of the previous 6 months which is to become
colD.

Speedy