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Homework Help: Poisson Distribution in excel

  1. Sep 25, 2013 #1
    1. The problem statement, all variables and given/known data
    Data from www.centralhudsonlab.com determined
    the mean number of insect fragments in 225-gram chocolate
    bars was 14.4, but three brands had insect contamination
    more than twice the average. Assume
    the number of fragments (contaminants) follows a Poisson
    1)If you consume seven 28.35-gram (one-ounce) bars this
    week from a brand at the mean contamination level, what
    is the probability that you consume one or more insect
    fragments in more than one bar?
    2)Is the probability of a test result more than twice the mean
    of 14.4 unusual, or can it be considered typical variation?

    2. Relevant equations
    Using EXCEL's POISSON.DIST Function.

    3. The attempt at a solution

    #1 is really confusing me. I find the wording to be very obscure.
    For a poisson distribution we can scale the mean to match the 28.35g bars.
    Thus our mean of interest is (28.35/225)*14.4=1.82

    My instructor told me that the answer is simply the (probability of consuming one or more insect fragments in ONE bar)^7.
    Does this make sense to you guys? Because it doesn't to me.
    To me the above is calculating the probability of consuming one or more insect fragments in SEVEN bars, rather than "more than one bar."

    #2 seems subjective. More than twice the mean is 28.8. I don't understand what they mean by "typical." Seeing that the problem statement said that 3 brands had contamination more than twice the average, I would assume it's typical, then again it depends on what you define typical as. Any suggestions>
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Sep 25, 2013 #2


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    Staff Emeritus
    Science Advisor
    Gold Member

    For number 1, I would use the fact that P(A happens) = 1-P(A does not happen).

    Number 2 is terribly worded. I think they mean something like "find the probability that a tested bar has at least twice as many insect fragments as the mean, and see if that number is really really small". The fact that you don't know how many brands were tested though makes this less than solid logic, but I assume this is what they are going for.
  4. Sep 25, 2013 #3
    Thanks for the reply.

    Regarding #1, yes, that is what I used, except my (1-P(A does not happen)) is equal to the probability that one is consuming one or more insect fragments in ONE bar. I am just confused on how to apply this to ONE OR MORE bars.

    The probability I calculated for #2 is 0.000463. I consider this to be really small, but I really don't understand what is the meaning of typical for this case.
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