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"It has long been an axiom of mine that the little things are infinitely the most important" 

Those margin totals
Therefore, to find what proportion (P) of occasions a given combination of cell frequencies can be expected to arise, you divide the number of ways you can achieve the margin totals (N_{t}) by the number of ways of obtaining the cell frequencies you found (N_{c}). P = N_{t} / N_{c} = {N!² / [(a+b)! (c+d)! a+c)! (b+d)!]} / {N! / [a! b! c! d!]} Rearranging this expression we get Cancelling the extra N! gives us
This is Fisher's standard formula.
