Having trouble w/ understanding pdf...

  1. 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.
     
  2. jcsd
  3. AKG

    AKG 2,585
    Science Advisor
    Homework Helper

    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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