I posted this question earlier and am rewriting it in hopes of getting some guidance. I am using a weighted regression (after propensity score matching) to obtain estimates of the effects of a treatment (treat) on an outcome for different income quintiles. I include interactions of the quintiles and the treatment in the equation, as shown below:
regress outcome treat quintile2 quin3 quin4 quin5 treatXquin2 treatXquin3 treatXquin4 treatXquin5
To obtain the effect of treatment on quintile2 I add the coefficient on treat with the coefficient on treatXquin2. However, what do I do with the standard errors? Can I simply add the standard errors on the two coefficients together? Can I add the t-stats together?
Any advice would be much appreciated. I'm not a stats expert but have a basic understanding of econometrics.