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Math Help - Quick Question on Probability and Normal Distribution tables

  1. #1
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    Quick Question on Probability and Normal Distribution tables

    When a question is of the form 'what is the probability that a project will take "at least" (suppose) 23 days', how do you find the probability using normal distribution tables?
    Last edited by DooBeeDoo; April 21st 2008 at 01:45 PM.
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    Super Member flyingsquirrel's Avatar
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    Hi

    Simply by using P(X\geq 23)=1-P(X\leq23) ?
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    Quote Originally Posted by flyingsquirrel View Post
    Hi

    Simply by using P(X\geq 23)=1-P(X\leq23) ?
    thanks. Also, how would you go about constructing a 95% confidence interval form (in my case) completion time of a project?
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    Quote Originally Posted by flyingsquirrel View Post
    Hi

    Simply by using P(X\geq 23)=1-P(X\leq23) ?
    You'd first need to convert X = 23 into a Z-score using the formula Z = \frac{X - \mu}{\sigma}.

    Then, depending on whether the z-score is less than 0 or greater than zero, there might be some further fiddling around needed before a value can be read from the standard normal tables .....

    If the Z-score is greater than zero, then you use 1 - Pr(Z < Z-score) since Pr(Z < Z-score) can be read straight from the table.

    However, if the Z-score is less than 0, you actually use Pr(Z < -(Z-score)). The following example illustrates the chain of logic that's required:

    Pr(Z > -1) = 1 - Pr(Z < -1) = 1 - Pr(Z > 1) = 1 - (1 - Pr(Z < 1)) = Pr(Z < 1).


    All this is assuming of course that you're using a normal distribution .....
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    Quote Originally Posted by DooBeeDoo View Post
    thanks. Also, how would you go about constructing a 95% confidence interval form (in my case) completion time of a project?
    It is a totally recipe driven process. Even I can do it! Where are you stuck?

    Look at this thread: http://www.mathhelpforum.com/math-he...y-answers.html

    for some worked examples. I'd be astonished if your class notes or textbook did not have worked examples.
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