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Question
I am trying to figure out how to calculate a probability using the function command in excel and i am drawing a blank.

I have a mean of 23.4,standard deviation of 9.05079 and am trying to get the probability of a number coming out between 17.2952 and 29.5048.

I thought Norminv function would work ,but no-good...........

I would appreciated any help,I am not a stat's person so,I could
be doing this backward altogether.....

Thx.

Joseph Z Ward

Answer
Josephjphs,


We know that on the standard normal distribution, the area between -1.96 and +1.96 includes an area (probability) of .95.  Recall that for the standard normal, the mean is zero and the standard deviation is 1.

so to test the approach I use the formula

=NORMDIST(1.96,0,1,TRUE)-NORMDIST(-1.96,0,1,TRUE)

so I get the cumulative area from minus infinity up to 1.96 and then subtract the cumulative area from minus infinity up to -1.96

I can use the same approach for you problem.


=NORMDIST(29.5048,23.4,9.05079,TRUE)-NORMDIST(17.2952,23.4,9.05079,TRUE)

give a probability of  0.50000947370046

since each of your end points are .674504656  from the mean, then this would include approximately the middle 50%.  

Note in Excel 2010, Microsoft introduced a standard naming convention for statistical functions.  While you can use the above, the new way would be:

=NORM.DIST(29.5048,23.4,9.05079,TRUE)-NORM.DIST(17.2952,23.4,9.05079,TRUE)

so there is a period between "NORM" and "DIST" =  NORM.DIST

I don't  have Excel 2010 installed here so I can't test that.  But if you run into problems, the old functions are still supported.

--
Regards,
Tom Ogilvy  
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Tom Ogilvy

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Selected as an Excel MVP by Microsoft since 1999. Answering Excel questions in Allexperts since its inception in 2001. Able to answer questions on almost all aspects of Excel's internal capabilities. If seeking a VBA solution, please specify that in your question itself so I give you the answer you want. [Excel has weak protection - if you are distributing an application, I don't answer questions on how to protect your project from your users.]

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