At 11:28 AM 1/15/2004 -0500, Wandland, Kristen wrote:
Thank you all for the information so far. I have yet to digest it
all!
I think I should help with a couple of clarifications: we are
looking at
potentially uneven sampling groups (n1 = 10, n2 = 15) and we do have
unequal
variances.
The ultimate task at hand is to determine if the concentrations (of
a
constituent) from one area are consistent with the concentrations
from the
other area. EPA has requested a ttest with diff = StDev.
In that case my clunky solution from before may not be so bad after all
(although somebody will no doubt suggest yet another better way to do it!).
I'm not sure what StDev you are referring to (the SD for Y with both groups
pooled together?) but anyway lets assume you have determined it is 10; and
lets further assume the hypothesis is one tailed in that you expect group 1
to be the group that scores higher. Then
gen score2 = score
replace score2 = score2-10 if group==1
ttest score2, by(group) unequal
You could modify this along the lines of what Scott suggested to use the
saved results from summarize. Does this sound like what you wanted?
-------------------------------------------
Richard Williams, Associate Professor
This is potentially exactly what I wanted. I need to try it all out, but in
syntax it looks like it'll work.
Thank you ALL for input!
Kristen Durocher Wandland
ENSR
7041 Old Wake Forest Road
Raleigh, NC 27616
919-872-6600
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