Hi,

I have a standard difference-in-differences setup where a reform is introduced in one region in a given year but none of the other regions. I’d like to estimate the effect of the reform across the distribution of the outcome variable. To do this, I am using the rifhdreg package, with the following code:

loc nreps = 1000
loc seed = 1019

glo controls “age gender income”

forval n = 1(1)99 {

bootstrap, reps(`nreps') seed(`seed'): rifhdreg outcome treat##post $controls, cluster(region) rif(q(`n'));

}

However, about 35% of the repetitions fail with the error message that “collinearity in replicate sample is not the same as the full sample, posting missing values”. See, for example, this output:

Array


It looks like that when re-sampling, it’s drawing samples without observations from all four groups that are needed to identify the parameters in the difference-in-differences model: (i) pre-reform control regions, (ii) pre-reform treatment region, (iii) post-reform control regions, and (iv) post-reform treatment region.

Is there a way to ensure that I'm drawing samples with observations from each of these four groups? If not, what is the best way to proceed?

Thanks in advance!