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!

Homework Help: Binomial distribution

  1. Jul 27, 2013 #1
    1. The problem statement, all variables and given/known data
    In a class with 20 and one with 10 students each student has a probability of 0.3 to not show up on a particular day. On a given day, which class is most likely to have the highest ratio of absent students? This was in my exam, unfortunately I did not know how to do it.

    2. Relevant equations
    The probability that n students are absent on a given day in one class is binomially distributed.
    I heard some say that since the relative standard deviation goes like 1/√N the class with fewer pupils is most likely to have most absent, but I am not sure about this.

    3. The attempt at a solution
    You could do a whole lot of counting, but I don't think that's the point. Besides it would be a very tedious proces. I did it with a class of 1 and 2 students and counting even then took time.
  2. jcsd
  3. Jul 27, 2013 #2


    User Avatar
    Science Advisor
    Homework Helper

    hi aaaa202! :smile:

    (i haven't actually tried this :redface:, but …)

    i think i'd calculate Pk, the probability that exactly k students out of 30 are absent

    and then for each k the probability Qk that less than one third of k from 30 come from the 10

    (and then multiply and add)
  4. Jul 27, 2013 #3


    User Avatar
    Homework Helper

    Let N be the number of students absent from the large class, and M the number of students absent from the small class. The ratio of absent students will be larger for the large class if N > 2M, equal if N = 2M, and smaller if N < 2M. So you need to work out the probabilities of those three events.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted