joint probability

Hi,

I need some instructoins about:

Given two larger vectors(A and B) with discrite time values, and considering that each vector was ordered previously.  how to compute the joint probability for each pair of values, considering the restriction that each element in A(Ai) is combined only with elements in B(Bi) that are lower than Ai? what is the more appropriate package/function to manipulate this kind of probability?

Thanks a lot 
Ricardo




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