Degrees of Freedom in TTest

[Guest]

According to http://office.microsoft.com/en-us/excel/HP052038731033.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

 

[Bernard Liengme]

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/archives/html/s-news/2005-08/msg00008.html
http://projectile.is.cs.cmu.edu/research/public/talks/t-test.htm
http://www2.hawaii.edu/~donaldp/Biostatitics/lecture10.htm

You will need to find a professional statistician to ask why there are two
methods.
best wishes