stdev is always positive. However:

mean+stdev
and
mean-stdev

may be negative if the mean is small or negative.
--
Gary''s Student - gsnu201001


"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.