J @ Y wrote:
Thanks for the solution. May I ask what your method of calculating quarterly
STdev is?

J@Y wrote previously:
My monthly data are in a column. I am currently using B1=Stdev(A1:A3)....
then selecting 3 vertical cells (ie: B1:B3) then dragging it down. But that
leaves each quarterly Stdev 2 spaces apart. How would I do it so that the
quarterly Stdev organized in 1 spaceless column?

If A1:A3 are "monthly data", then stdev(A1:A3) is the "monthly std dev"
-- that is, the std dev of the monthly data, not the quarterly data.
If you want a quarterly std dev, you need "quarterly data". Exactly
what that means depends on what your "monthly data" are. For example,
if they are the number of sales per month, then for quarterly
statistics, I would sum every 3 months and take the std dev of the
quarterly sums. If the "monthly data" are growth rates, I would
compute the 3-month growth rate and take a std dev (arithmetic or
geometric) of the quarterly rates.

Moreover, you are computing a std dev of only 3 data points. Usually
that is not sufficient data to determine a std dev -- that is, a std
dev that you intend to use for other statistical inferences.
Arguably, the minimum number of data points needed depends on the size
of the confidence interval that you want around the average and on the
dispersion of the data. (Yes, the std dev is a measure of dispersion;
so yes, that is a circular specification -- intentionally so.) So 3
data points might work in some circumstances. But usually not.

HTH.