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: Writing probability functions

  1. Oct 21, 2012 #1
    Suppose that a fiat coin is tossed 10 times independently. Determine the pf of the number of heads that will be obtained.

    I'm feeling really out of touch with this material. It seems like any number of heads is equally likely but I know that just can't be right.

    My reasoning for saying such is that the probability of a head is 1/2 and the probability of a tail is 1/2,

    So the probability of a head, then tail, then head is


    Or head, head, head is


    But I know that getting 5 heads in the ten flips and getting all heads aren't equally likely.. are they?

    So can someone "remind" me how my thinking is wrong?
  2. jcsd
  3. Oct 21, 2012 #2
    Wait, I think I see. The probability of any order of heads or tails is equally likely, but the number of "ways" 5 heads can occur is more than the number of ways 10 can occur (which is just one.)


    f(x) = (10 CHOOSE x)(1/2)^10

  4. Oct 21, 2012 #3


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Yes. You might want to review the binomial distribution.
  5. Oct 21, 2012 #4
    Indeed. Should I also make the function piecewise, 0 for any exception to the integer interval 1 through 10?
  6. Oct 21, 2012 #5
    Since each trail can be classified as a simple success or failure, we use the binomial distribution, as you have already written down for this case. Just incase, the general form is:

    [itex] P(X) = C^{n}_{x}p^{x}q^{n-x} [/itex]

    p = probability of success in a trial
    q = probability of failure = (1-p)

    Edit: Yes, the distribution is inherently discontinuous at non-integer values since there can only be an integer number of successes in n trails.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook