I want to run a diff-in-diff model. To choose an appropriate control group, I use a nearest-neighbor matching model based on several determinants of the outcome variable that I study.
I was wondering: is it possible to directly match my observations using the values of the outcome variable before the treatment instead of simply using the determinants? It seems like a perfect way to obtain a parallel trend for the outcome variable for the two groups in the pre-treatment period. But most papers I read avoid matching directly on the outcome variable in the pre-treatment period, there must be an explanation.