# Thread: Need help with Poisson Distribution

1. ## Need help with Poisson Distribution

Hi Everyone!

I`ve been really stuck on a biostats question that just doesn`t seem to make sense. The work I have is

Pr(X) = (e^-u) (u^x) / X!

Pr(1) = (2.718^-0.65) (0.65^1) / 1! = 0.339

Pr(0) = (2.718^-0.65) (0.65^0) / 0! = 0.522

Pr(2) = (2.718^-0.65) (0.65^2) / 2! = 0.110

Pr(3) = (2.718^-0.65) (0.65^3) / 3! = 0.048

Pr(>3) = 1 – [(Pr3)+(Pr2)+(Pr1)+(Pr0)] = 1 –

Pr(3) is what is getting me as everytime I do it I get 0.048 but the answer is supposed to be 0.024. I just don`t get why the answer is 0.024 as I used the exact same method to calculate the other Pr(X)`s and got the correct answer but for some reason this one just ins`t right. I checked online at two different sources which said the answer is 0.048 so I don`t think its a miscalculation.

Thanks!

2. ## The Following User Says Thank You to Opendome For This Useful Post:

hlsmith (10-17-2015)

3. ## Re: Need help with Poisson Distribution

A computing error:
Pr(3) = (2.718^-0.65) (0.65^3) / 3! = 0.048
I paste it in in google and get
(2.718^-0.65) (0.65^3) / 3! =
0.02389608065

and in R

Code:
``````> (2.718^-0.65)*(0.65^3) /factorial(3)
[1] 0.02389608
> exp(-0.65)*(0.65^3) /factorial(3)
[1] 0.02389447``````

4. ## The Following User Says Thank You to GretaGarbo For This Useful Post:

Opendome (10-18-2015)

5. ## Re: Need help with Poisson Distribution

Originally Posted by GretaGarbo
A computing error:

I paste it in in google and get
(2.718^-0.65) (0.65^3) / 3! =
0.02389608065

and in R

Code:
``````> (2.718^-0.65)*(0.65^3) /factorial(3)
[1] 0.02389608
> exp(-0.65)*(0.65^3) /factorial(3)
[1] 0.02389447``````
Thanks for the help! I was jsut wondering why 3! = 6?

6. ## Re: Need help with Poisson Distribution

I was jsut wondering why 3! = 6?
If you are not sure about this I suggest you to read more about the definition of factorial (the ! sign) before proceeding in this question. And in general this will be important as you will face it in combinatorics.

7. ## Re: Need help with Poisson Distribution

Thanks! That's exactly what I was confused about

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