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Basic stats questions

  1. Sep 10, 2006 #1

    This problem sounds really easy, but I think I am doing something wrong.

    Question :

    If the mean time between some random event occuring is 6 months, what is the probablity that in one year the event does not happen.

    I think its like flipping a coin. There is a 0.5 chance of the event NOT happening in 6 months, so there is 0.5 x 0.5 chance of it not happening in 1 year, so P = 0.25.

    Does this sound right?

  2. jcsd
  3. Sep 11, 2006 #2
    That could not be in generarl correct: take the case of a baby averaging nine months between conception and delivery. Time difference would be pretty much restricted by this 9 month figure.

    And even if someone was to argue that what in medicine is "normal," is not the same as the "mean," well take the "normal temperature" of 98.6F; if you took the temperature of 10,000 people, I am sure the mean result could not vary much from that, probably by less than 2 degrees F.
    Last edited: Sep 11, 2006
  4. Sep 11, 2006 #3
    What you have a classic case of an exponential distribution with 6 months as the rate parameter. This should be enough for you to figure the problem out.
  5. Sep 11, 2006 #4
    Thanks for the leeds guys.

  6. Sep 16, 2006 #5
    how does this look:

    rate =

    [tex] e^{-\frac{\labmda}{t}}=e^{-\frac{12}{6}}[/tex]
    probability of event occuring in one year = 1-rate =0.865
    Last edited: Sep 16, 2006
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