Has anyone used a Wald Test to check for parallel trends as a pre-testing stage for a Difference-in-Difference? Had been advised that it is a good way to do so, but not sure of the best way to undertake.
I have a dataset consiting of 13 pre-policy observations for both a subject and control - coded '1' for treatment and '0' for control. Does anyone have any code/has anyone undertaken such in R.