I have a multivariate regression, which includes interactions. For example, to get the estimate of the treatment effect for the poorest quintile I need to add the coefficients from the treatment regressor to the coefficient from the interaction variable (which interacts treatment and quintile 1). When adding two coefficients from a regression, how does one obtain standard errors? Is it possible to add the standard errors from the two coefficients? What about the t-stats? Is it possible to add these as well? I’m guessing not but I can’t find any guidance on this.

Thanks so much in advance for your help!

**Answer**

I think this the expression for SEbnew:

√SE21+SE22+2Cov(b1,b2)

You can work with this new standard error to find your new test statistic for testing Ho:β=0

**Attribution***Source : Link , Question Author : Sarah , Answer Author : suncoolsu*