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Poisson distribution question

  1. Aug 2, 2010 #1
    Hey guys, I'm kind of stuck on this question.

    In a certain town, crimes occur at a Poisson rate of 2.4 per month (i.e. according to a Poisson process with a rate of 2.4 per month). What is the probability of having exactly 2 months (not necessarily consecutive) with exactly 4 crimes during the next year? Assume that every month has the same length.

    I know that first you have to find the probability of 4 crimes in 2 months using Poisson distribution then use binomial distribution to answer the question. But i'm not sure how to do the first part! >.<
  2. jcsd
  3. Aug 3, 2010 #2
    If the rate is 2.4 per month, then the probability of a crime happening in any given month = 1 - (1/2.4) (or so I think) = 58.33% approximately. Then the probability of four crimes in two months = the probability of exactly one crime in two months^4.
  4. Aug 3, 2010 #3


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    Poisson chance of 4 crimes in a month is
    [tex]\frac{2.4^4e^{-2.4}}{4!}[/tex]. Use binomial from here.
  5. Aug 3, 2010 #4
    I don't think this would work as the question is asking for the probability of exactly 4 crimes occurring in 2 months.
  6. Aug 3, 2010 #5
    This is correct. You do mean 4 crimes occuring in each of 2 months right? If not you need to word the question more clearly.
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