Andrew,

I see! yes that makes sense.  Thank you for explaining.

3 additional questions:
1) you note that the F-test will work for the positive contrast and t-test will work for either.  Is this because a t-test and f-test are essentially the same thing when comparing just 2 groups?  And a negative contrast doesn't make sense in the f-test because it measures variance?

2) I have unequal sample sizes for the 2 groups.  one group has 33 patients and the other has 57.  Will I need to adjust the model?

3) I have some nuissance variables to also add.  Can put them in the last column with a corresponding contrast of '0'?

Thanks again,
J