Dear Listers,

I am looking into the difference between testparm and contrast following a regression model. Reading online, it seems that testparm computes a Wald test which determines whether the variable improves the model while contrast produces comparisons akin to ANOVA tests.

I am interested in the impact of different therapies on anxiety scores. I have three treatments I am comparing but to get my head around what is happening I start with comparing 2 of them only and I adjust for baseline scores of anxity. My model looks like this in Stata 16:

regress anxiety i.treatment anxiety_base

The regression coefficient for treatment is 0.69 (95%CI:-0.41 to 1.85), p = .22

if I type testparm i.treatment, I get

F(1, 134) = 1.46, p = .22 which is in line with the regression coefficient

when I type contrast i.product (or r.product which eventually I plan to use when comparing all 3 treatments), I get: F(1) = 7.6, p = .007

why is the contrast so different and what approach should be used to compare contrasts between specific treatments? Feel free to point me to any relevant resource.

Thanks in advance