Help finding percentage of data
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 This topic has 4 replies, 3 voices, and was last updated 16 years, 9 months ago by Sathiya.

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February 28, 2006 at 4:49 pm #42552
if i given the problem to find the percentage of data that falls between plus/minus 2.3 standard deviation from the mean, how do i determine this? I look at z chart and see low numbers but i beleive answer should be something around 95%.
am i not using correctly?0February 28, 2006 at 5:01 pm #134406A typical Z table is giving you the % beyond the zvalue not the % between the mean and the Z value.
So.. if you look up a Zvalue of 2.3 you get .01072 or 1.07% right? So, you need to subtract this value from 1 to the percentage under the Zvalue. 1.01072 = .9892.
So.. if you have 1.07% on one side, what percentage would you have on the other side? What is left?0February 28, 2006 at 5:47 pm #134409thank you for your answer but i am still confused. if i look at 2.3 on the z table, i get .04893. i feel i am looking at wrong table? i search this site and found one that looks just like mine with same answer.
0February 28, 2006 at 6:06 pm #134410No.. it is .4893 which is a lot different then .04893. That is a different Ztable which is giving you the % between the mean and the ZValue.
So, all you need to do is double the number. .4893*2 = .9786 or 97.86% is between +/ 2.3.
You would get the same answer using my other post right? 1.07*2 = 2.14. 1002.14 = 97.86!
Magic!
I hope that clears it up. The differnce was the Ztable. Most Ztables in stats books will not be like the one you found on this site.
Cheers!0March 3, 2006 at 5:11 am #134573Just now i have gone through your doubt.
To find area under the curve, it is enough to use normdist funtion in excel.
The function is normdist(X, mean, stdev, cumulative).
In your case it is normdist(2.3, 0,1,1) = 0.9892, which is 98%. So 98% of your data falls within +/2.3 stdev.
regards0 
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