+ Reply to Thread
Results 1 to 5 of 5

Thread: Need help with Poisson Distribution

  1. #1
    Points: 272, Level: 5
    Level completed: 44%, Points required for next Level: 28

    Posts
    25
    Thanks
    7
    Thanked 1 Time in 1 Post

    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. #2
    Human
    Points: 12,666, Level: 73
    Level completed: 54%, Points required for next Level: 184
    Awards:
    Master Tagger
    GretaGarbo's Avatar
    Posts
    1,360
    Thanks
    455
    Thanked 462 Times in 402 Posts

    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
    Please note that 3! =6

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

    Opendome (10-18-2015)

  5. #3
    Points: 272, Level: 5
    Level completed: 44%, Points required for next Level: 28

    Posts
    25
    Thanks
    7
    Thanked 1 Time in 1 Post

    Re: Need help with Poisson Distribution

    Quote Originally Posted by GretaGarbo View Post
    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
    Please note that 3! =6
    Thanks for the help! I was jsut wondering why 3! = 6?

  6. #4
    TS Contributor
    Points: 22,410, Level: 93
    Level completed: 6%, Points required for next Level: 940

    Posts
    3,020
    Thanks
    12
    Thanked 565 Times in 537 Posts

    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. #5
    Points: 272, Level: 5
    Level completed: 44%, Points required for next Level: 28

    Posts
    25
    Thanks
    7
    Thanked 1 Time in 1 Post

    Re: Need help with Poisson Distribution


    Thanks! That's exactly what I was confused about

+ Reply to Thread

           




Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts






Advertise on Talk Stats