Dear all,

I am researching the effect of grade retention on exam results (which can vary from 0 to 20) and I am using a RDD to research the LATE. However, when I use the rdrobust command, it states that it is not possible to compute the local polynomial bandwidth and that I should run rdbwselect () for more information.

After running the rdbwselect-command, Stata says: Invertibility problem in the computation of preliminary bandwidth below the threshold. Invertibility problem in the computation of preliminary bandwidth above the threshold. Invertibility problem in the computation of bias bandwidth (b) below the thresholdInvertibility problem in the computation of bias bandwidth (b) above the threshold. Invertibility problem in the computation of loc. poly. bandwidth (h) below the thresholdInvertibility problem in the computation of loc. poly. bandwidth (h) above the threshold

I think the problem is that my running variable as well as my outcome variable are discrete variables (it's either 0, 1, 2, ..., 18, 19, 20). Is there any way I can solve this?

Kind regards,
Charles Calmeyn