# Having trouble w/ understanding pdf...

by semidevil
Tags: trouble, w or
 P: 143 so I"m trying to understand pdf, the one where the formula is ((rCk) * (wCn-k))/(NCn). Here, xCx means x chooses x. anyways, N is the total, and n is how much you sample out of it. and we know that r + w = N. what is k? I know this is diffenrent for all problems, but is there an analogy..I"m having trouble setting things up.
 Sci Advisor HW Helper P: 2,586 When you say, "the formula is ..." doesn't it strike you as odd that there is no "=" sign. What is a formula without that sign? What you have is an expression for the probability of getting k of the things which there are r of in total. If "r" represents the number of red balls in total, and "w" represents the number of white balls, then: $$P(k) = \frac{{r\choose k} {w\choose {n - k}}}{{N\choose n}}$$ However, this only holds for max{0, n – w} ≤ k ≤ min{n, r}. Clearly, the probability is zero otherwise.

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