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# Pert Analysis - Normal Probability Tables

Posted on 2009-02-19
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Hello,

I'm doing some work on PERT Analysis.  In the example that I've been given it says to multiply the standard deviation by a value from the normal probability table, in the example given it states 1.96 will give 2.5% chance of going over.

Ive looked at the normal probability table and cannot see how the figure 1.96 or 2.5% is created?

http://209.85.229.132/search?q=cache:7Xz8ZLdXI_8J:www.stat.psu.edu/~babu/415/norm-tables.pdf+normal+probability+table&hl=en&ct=clnk&cd=1&gl=uk

Can anybody explain how the percentage figure is derived from the value?

Thanks.
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Question by:andyw27

LVL 58

Accepted Solution

Look at the row 1.9, skid over to column 0.06, you will read 0.97500. This means that P(X<1.96) = 0.975, or 97.5%. Conversely, P(X>1.96) will be 0.025, or 2.5%. In plain English, if a variable X follows a normal distribution of mean 0, and standard deviation 1, each draw will have only a 2.5% probability of being over 1.96.

Does that help?
(°v°)
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