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
