I have a dataset with dichotomous variables that has missingness that I am trying to use Amelia to address. In addition, I need to include observational priors (ridge priors are not an option).

Given the data I am using, I originally tried to run Amelia with these dichotomous variables identified as "noms". However, I got the follwoing error message when I did this:

According to the documentation (footnote 8 on page 17): "Dichotomous (two category) variables are a special case of nominal variables. For these variables, the nominal and ordinal methods of transformation in Amelia agree." So, I was particularly confused by this error message given that I had already tried running the exact same code with the only difference being that I had used "ords" to identify my dichotomous variables. And that code had worked.

For now, I have the code running using using the "ords" option instead of the "noms". I can only assume that the introduction of priors is causing the problem. It is not clear to me why this should be, though. Also the fact that, contrary to the documentation, I am not getting the same results for the ordinal and the nominal methods makes me wonder if the two methods are not actually identical, in which case maybe it does matter which option I use (beyond, of course, the matter of which method actually runs!).

I would appreciate any guidance you can offer as to what is causing this error message. I have attached code, and the resulting output, replicating the problem with the freetrade data, in case that helps clarify the issue.