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Geometric Distribution Coin Flip

  1. Oct 11, 2011 #1
    Consider the following experiment: a coin that lands heads with
    probability p is flipped once; if on this first flip it came up H, it is then repeatedly flipped until a T occurs; else, if on the first flip it came up T, it is then repeatedly flipped until a H occurs. Let X be the total number of flips. Find the p.m.f. of X, and the mean of X.

    Ok so I know this has to be a geometric distribution because we will repeat the experiment until we get a different side of the coin.

    Probability h = p and t = (1-p);
    Does this mean you have to condition for the two different cases and sum them their probabilites?

    Aka Px(k|1st T) = p* (1-p)^(1-k)

    and Px(k|1st H) = (1-p)*(p)^(1-k)?
  2. jcsd
  3. Oct 11, 2011 #2

    Ray Vickson

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    Yes, it means that; however, you need (1-p)^(k-1) and p^(k-1), not what you wrote.

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