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Stats: finding probability in normal distribution

  1. Jun 2, 2008 #1
    1. The problem statement, all variables and given/known data

    Find Z0 such that P(z > z0) = 0.1234

    2. Relevant equations

    3. The attempt at a solution

    Z is the mean which is 0. So if Z0 is less than the mean it should be a negative number. Looking at the table 0.1234 does not show up but the closest is 0.1217 which is 0.31.
    So Z0 is -0.31?
  2. jcsd
  3. Jun 2, 2008 #2
    It depends on if you're saying from [itex]-\infty \leq z \leq z0[/itex] or [itex]0 \leq z \leq z0[/itex]. The value you found is for the latter (+0.31). It sounds like intuitively you are thinking of the former case, but used the table for the latter case.

    Here if you needed another table-- http://www.math.unb.ca/~knight/utility/NormTble.htm
  4. Jun 2, 2008 #3


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    Z is not the mean, it is the standard normal random variable. You want to find P(Z > z0). So you want to find P(Z<z0) such that it equals to 1-P(Z>z0). 1-0.1234=0.8766, which corresponds to about z0=1.155 from the table David posted
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