David,
Most books use the simple df= n1+n2-2
However, look at these for references to the more complex formula
http://www.biostat.wustl.edu/archive.../msg00008.html
http://projectile.is.cs.cmu.edu/rese...lks/t-test.htm
http://www2.hawaii.edu/~donaldp/Bios.../lecture10.htm

You will need to find a professional statistician to ask why there are two
methods.
best wishes
--
Bernard V Liengme
Microsoft Excel MVP
www.stfx.ca/people/bliengme
remove caps from email


"David B" wrote in message
...
According to http://office.microsoft.com/en-us/ex...038731033.aspx
degrees of freedom in t-test for two unequal variances (regardless of
using
TTEST or t-test from toolpak) uses a formula where
(S1^2+S2^2)^2(((S1^2/m)^2/m-1))+((S2^2/n)^2/(n-1))) = DF (call this
formula
"Formula 1").

Normally, DF is simply equal to count of sample1 + count of sample2 - 2
("Formula 2"). Is there any reason why Excel uses Formula 1? I have
searched
for Formula 1 in a number of statistics manuals and have been unable to
find
an example of it.

Best,
David