3x the Standard Deviation Creates Negative Intervals

JohnnyCash

New Member
HI,

I have collected data in excel that i will be using for my thesis research. For many of the data sets, the standard deviation in the third interval is above 100 and below 0, though no numbers in any of these sets are below 0 or above 100.

For example,

The mean of one data set is 40.33 and the standard deviation is 21.29...meaning its extends from about -24-104, but this makes no sense to me as no number is >100 or <0. I want to create a normal distribution graph, using the data points I collected, in order to find data points that are at the extremities of the distribution to compare them. I have looked online for days and have tried everything. Is this an input error or does -24-104 make sense and if it does how? I just want a standard deviation that can be multiplied 3 times and is still less than the mean so to have a positive range.

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tusharm

MrExcel MVP
A normal distribution can have values from -infinity to +infinity, though the values at the very extremes will occur with very small probabilities. +/- 3 sigma contains 99.73% of the values in a normal distribution. So, in your case, if you look at the population that your sample represents, 0.27% of the values will be either < -24 or > +104.

It's the nature of a ~N distribution. If that doesn't meet your dissertation needs you should explore other distributions. Check with your advisor(s).
HI,

I have collected data in excel that i will be using for my thesis research. For many of the data sets, the standard deviation in the third interval is above 100 and below 0, though no numbers in any of these sets are below 0 or above 100.

For example,

The mean of one data set is 40.33 and the standard deviation is 21.29...meaning its extends from about -24-104, but this makes no sense to me as no number is >100 or <0. I want to create a normal distribution graph, using the data points I collected, in order to find data points that are at the extremities of the distribution to compare them. I have looked online for days and have tried everything. Is this an input error or does -24-104 make sense and if it does how? I just want a standard deviation that can be multiplied 3 times and is still less than the mean so to have a positive range.

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