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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

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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