1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Help with negative binomial distributions

  1. Mar 25, 2010 #1
    One of the questions in my probability homework reads:

    X denotes a negative binomial random variable, with p = 0.6 Find P(X ≥ 3) for a) r = 2 and b) r = 4.

    According to my teacher, the answers are 0.1792 and 0.45568, respectively, but I can't for the life of me figure out how he got them. I tried finding P(X ≥ 3) by turning it into 1 - P(X ≤ 2) and then calculating p(2), p(1), and p(0), but I kept getting 0 for my answer, which obviously isn't correct.

    Can someone please help me solve this problem, or explain to me how I would go about solving it? I'm really confused.

    Thanks.
     
  2. jcsd
  3. Mar 25, 2010 #2

    LCKurtz

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Remember that the negative binomial models the number of Bernoulli trials up to and including the rth success. Therefore [itex]p(x) > 0[/itex] only for [itex]x \ge r[/itex].

    If r = 2, then p(0) and p(1) are obviously zero, so your first calculation should just be 1 - p(2), which isn't zero and isn't his answer either. And if r = 4, obviously P(X ≥ 3) = 1 since you can't have 4 successes in less than three trials. Time to ask your teacher what's going on.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Help with negative binomial distributions
  1. Binomial distribution. (Replies: 1)

  2. Binomial distribution (Replies: 12)

Loading...