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Having trouble w/ understanding pdf...

by semidevil
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Oct26-04, 11:59 AM
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.
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Oct26-04, 12:32 PM
Sci Advisor
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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:

[tex]P(k) = \frac{{r\choose k} {w\choose {n - k}}}{{N\choose n}}[/tex]

However, this only holds for max{0, n w} ≤ k ≤ min{n, r}. Clearly, the probability is zero otherwise.

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