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...
--
HTH...

Jim Thomlinson


"PMK" wrote:

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.