Negative Standard Deviations?

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PMK

I have a spreadsheet with 52 data points on which I'm trying to identify a +1
standard deviation and a -1 standard deviation. When I use the Excel 2003
std dev function to calculate the info, I get a negative standard deviation
on the -1 calculation. Not being a stats wizard, I think it's mathematically
impossible for a standard deviation to be negative. I am assuming I have a
error in my data or formula. My data has a wide variation from a few around
1500 to many 0 and some data in between. Any thoughts would be greatly
appreciated.
 
StdDev is a measurement of the variablility of the data so you are correct in
assuming that there is no such thing as a negative StdDev. At 1 stddev about
2/3 of your data points will lie within 1 std dev of the average. 2 std dev
is around 95% of the data points. So the avearge temperature in January is 5
degrees +/- 3 degrees 67% of the time means that the temp is going to be
between 2 and 8 degrees most days. I would never calculate the -std dev as it
will be the same as the positive. Not sure if that makes sense...

Post your data pints and your formulas if you are still having trouble...
 
stdev is always positive. However:

mean+stdev
and
mean-stdev

may be negative if the mean is small or negative.
 
Jim Thomlinson said:
At 1 stddev about 2/3 of your data points will lie within 1 std dev of
the average.
Click to expand...

That is the expected result for a normal distribution. It may or may not be
true for other distributions. As you say, the standard deviation is a
measure of variability from the mean. Its use is not limited to normal
distributions.


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