Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Probability and binomial distribution question

  1. Mar 23, 2006 #1
    There was a question on the test with the following information (binomial distribution)

    n=10
    p=.2

    Find the probability that X is :
    a. At least 3
    b. At most 3

    For part a I did P(X>=3)=1-P(X<=2)

    For part b I did P(X<=3) :
    [itex] \sum_{x=0}^3 perm(n, x)*p^x*(1-p)^(n-x)[/itex]

    The last part is (1-p)^(n-x)
    For part b, I got an answer that is greater than 1, what did I do wrong?
     
    Last edited: Mar 23, 2006
  2. jcsd
  3. Mar 23, 2006 #2

    Tide

    User Avatar
    Science Advisor
    Homework Helper

    Why are you using "perm?"
     
  4. Mar 23, 2006 #3
    Disregard. How do you delete your posts? I pressed the delete button once I go to edit, but that doesn't do it...

    Anyways, I got an answer of .7717.
     
    Last edited: Mar 23, 2006
  5. Mar 23, 2006 #4
    Perm meaning permutation. That's the formula for binomial distribution. Ksinclair, should I disregard your answer?
     
  6. Mar 23, 2006 #5
    Probably :P

    I know that moderators can delete posts, but I thought that you could also delete them yourself, or else my post wouldn't even be up there right now.

    If perm (n, x) means n!/(x!(n-x)!), then my answer should be correct ;).
     
    Last edited: Mar 23, 2006
  7. Mar 23, 2006 #6
    Wow, this sucks. I wrote it out right on the paper but when I kept calculating I kept switching the x and n-x place so i had a greater answer. I can't believe this, I calculated it like 5 times on the test and still didn't realize the mistake!
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook