using data till till the current row

Hi All,
here is what I would like to do :-
Suppose I have the following :-

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So essentially p = (# 1 in type until that row) / (total # of types until that row) .
I have calculated the p's manually. But for a very large data set, thats just not feasible.
Can anyone suggest how to use any function or program to do that in excel 2007 ??????
Thanks

Assuming type starts in A2, in B2 and copy down,

=COUNTIF(A$2:A2, A2) / ROWS(A$2:A2)