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Why is my lamda different from the formula used by the book in Poisson Distribution?

  1. Sep 25, 2011 #1
    1. The problem statement, all variables and given/known data
    A store selling newspapers orders n = 4 of a certain newspaper because the manager does not get many calls for that publication. If the number of requests per day follows a Poisson distribution with mean 3, what is the expected value of the number sold?

    2. Relevant equations
    Now we are approximating this with a Poisson distribution as said. So I would think to use 3 because our book explicitly states mean (mu) = sigma^2 = lambda.


    3. The attempt at a solution
    Thus, my thought is that lamba should be 3 which, I thought should be the expected value. However, I also know through moment-generating functions that the sum of x*P(X=x) is also my expected value. But if I use a lambda of 3, I still can't get the books answer of 2.681.

    Thanks for your help!
     
  2. jcsd
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