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Homework Help: Binomial Distribution Statistics Problem

  1. Jan 17, 2009 #1
    1. The problem statement, all variables and given/known data

    Estimate the probability that, in a group of five people, at least two of them have the same zodiacal sign. (There are 12 zodiacal signs; assume that each sign is equally likely for any person.)

    2. Relevant equations

    P(X=k) = nCk * [tex]p^{k}[/tex] * [tex](1-p)^k{}[/tex]

    3. The attempt at a solution

    I think that it involves in some way subtracting the probability that everyone has a unique zodiacal sign from 1, but I'm not sure exactly how.
  2. jcsd
  3. Jan 17, 2009 #2
    Could you possibly tell us a little more about the variables in that formula. "What do C, k, n, and p represent?"
  4. Jan 17, 2009 #3
    Look up the birthday problem, it's pretty similar to this isn't it?
  5. Jan 17, 2009 #4


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    Homework Helper

    Exactly the right approach. How many ways are there to assign a different sign to each person and how many ways to assign any sign to any person. Take the ratio and subtract from 1.
  6. Jan 18, 2009 #5


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    Your formula is incorrect- perhaps a typo. It should be
    P(X=k) = nCk * [tex]p^{k}[/tex] * [tex](1-p)^{n-k}[/tex]

    Yes, "at least two the same" is the opposite of "all different". Since there are 5 people and you want 5 different signs, both n and k in your binomial coefficient are 5 so that is easy- its just 1. In fact, [itex](1- p)^{5-5}= (1-p)^0= 1[/itex] so it is just "probability of all the same" is [itex]p^5[/itex]. What is p?
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