# Thread: verification of evaluation of a formula

1. ## verification of evaluation of a formula

Calculating a poisson probability one uses the formula (at least at this point in my class)

e^(-y) times (y^x)/(x!)

where x is the number of occurrences and y is the average number of occurrences per time period.

for x=10 and y=5 I get

e^(-5)*((5^10)/(10!))

e^(-5) = 0.00673794699
5^10 = 9765625
10! = 3628800

so we have 0.00673794699 times (9765625/3628800)
becomes 0.00673794699 times 2.69114445547
which gives 0.01813278868 or 0.0181 to four decimal places
But the correct answer is supposed to be 0.0137

where did I go wrong?

2. ## Re: verification of evaluation of a formula

You didn't do anything wrong. 0.0181 is correct.