Having trouble w/ understanding pdf

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SUMMARY

This discussion focuses on understanding the probability formula for sampling from a population, specifically the expression P(k) = (rCk * wCn-k) / NCn. Here, 'r' represents the total number of red items, 'w' represents the total number of white items, and 'N' is the total number of items. The formula calculates the probability of selecting 'k' red items when sampling 'n' items from the total population. The discussion highlights the importance of understanding the constraints on 'k', which must satisfy max{0, n – w} ≤ k ≤ min{n, r} for the probability to be valid.

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semidevil
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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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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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