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Math : Binomial + Poisson

  1. Feb 14, 2016 #1
    1. The problem statement, all variables and given/known data
    I need to find the variance of Y.

    The number of errors on a page follows a Poisson distribution with lambda = 0.40 average . Y = the number of pages without error among the first 112 pages .

    2. Relevant equations


    3. The attempt at a solution
    In Poisson, I know that Variance = lambda= 0.40
    In Binomial, Variance = n*p*q
    1-p = q
    Do I simply do 112 x 0.4 x 0.6 to find the variance of Y? How am I supposed to do it?
     
  2. jcsd
  3. Feb 14, 2016 #2

    andrewkirk

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    First use the Poisson to work out the probability of a page having no errors. Then use that number in the Binomial distribution to solve the problem.
     
  4. Feb 14, 2016 #3
    I got this resolved by doing

    poissPdf(0.4,0) =p
    n = 112

    variance with binomial = n*p*q
    and its correct answer :)
     
    Last edited: Feb 14, 2016
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