I have a panel dataset where I am trying to estimate the effect of receiving a grant on publication. I had hoped to use differences-in-differences estimator, but the problem is that many people receive grants multiple times over the course of the dataset, so they aren't simply pre- and post-treatment. I have not been able to find a technique for dealing with treatments that occur to the same person multiple times. I also can't use a matched sample because people who have never received grants are not similar to those who have. Does anyone know of a technique?