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  1. #1
    3 Star Lounger
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    CALULATION OF POISSON PROBABILTY (97)

    To calculate the probability of that 120 back aches in a hospital is really different that the average number of the last 4 years which is a 100 i use = 1-poisson (120-1,100,true) which gives a correct answer of 2,8 % . However I cannot not get a number if the numbers are 10 times that level for = 1-poisson (1200-1,1000,true) . My answer is and I quote #number!. I would like to compute the p for all 10 hospitals. What are the computational limitations on using the poisson function.

  2. #2
    Plutonium Lounger
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    Re: CALULATION OF POISSON PROBABILTY (97)

    The formula for the Poisson distribution involves factorials; the largest number for which Excel can compute the factorial is 170. Excel probably doesn't use the literal formula, but some kind of approximation; still, numbers like 1000 cause an overflow.

    Added later:

    For large numbers, the Poisson distribution can be approximated by a normal distribution with the same mean, and with the square root of the mean as standard deviation. So =1-NORMDIST(B3-1;B2;SQRT(B2);TRUE) would be a good approximation of =1-POISSON(B3-1;B2;TRUE). See attached modified spreadsheet.

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