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?

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- Apr 21st 2008, 11:17 AMDooBeeDooQuick 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? - Apr 21st 2008, 11:20 AMflyingsquirrel
Hi

Simply by using $\displaystyle P(X\geq 23)=1-P(X\leq23)$ ? - Apr 21st 2008, 12:47 PMDooBeeDoo
- Apr 21st 2008, 04:40 PMmr fantastic
You'd first need to convert X = 23 into a Z-score using the formula $\displaystyle Z = \frac{X - \mu}{\sigma}$.

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 ..... - Apr 21st 2008, 04:46 PMmr fantastic
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.