Allow matching on list variables

[kosukeimai]

Allow matching on list variables

[ngreifer]

I like the idea of a partial matching by at least one element in a list variable, but I'm not inclined to add that into MatchIt because list variables do not play a clear role in computing a distance metric such as the propensity score difference or Mahalanobis distance and adding support for them would require a complicated restructuring of the code and specialized routines for what is a very uncommon problem. MatchIt is not designed for highly customized matching specifications like the one you described, but you might be able to implement a version of it in Matching or optmatch by manually restricting certain matches.

In Matching, you could use the restrict argument in the call to Match() after computing the restriction matrix, which you would have to do manually by looping through each treated and control unit. In optmatch, you could create a distance matrix using match_on() and then manually set distance between units that do not share a value of the list variable to Inf to prevent them from being matched. You would then supply that matrix to fullmatch() or pairmatch().