"Jim Thomlinson" wrote:
At 1 stddev about 2/3 of your data points will lie within 1 std dev of
the average.

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.


----- original message -----

"Jim Thomlinson" wrote in message
...
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.