Hello, I'm the mainter of the package *MatchIt*, which uses *Rcpp* to implement nearest neighbor matching. One way to customize nearest neighbor matching is to add a caliper, which is the largest distance two units can be from each other before they are not allowed to be matched. I've had some users complain recently that the caliper is not working for them, i.e., even after specifying a caliper, units are being matched who are farther apart then the caliper width. I have been unable to replicate this problem on my Mac; the caliper always works as intended.
One user noted that even when using the same package version, the performance varied across two machines: one obeyed the caliper and one didn't. I thought this might be related to the version of R installed, as for some users updating R fixed the issue, but for others it didn't. I'm kind of at a loss. I have a suspicion that the problem is related to the function abs() in the C++ functions find_right() and find_left() that I wrote to perform the matching, which are in nn_match_vec.cpp <https://github.com/kosukeimai/MatchIt/blob/master/src/nn_matchC_vec.cpp>. I have had problems with abs() before (seemingly related to a namespace conflict between std and Rcpp). In this case, abs() is used in the following way: if (abs(distance[ii] - distance[k]) > caliper_dist) { //if closest is outside caliper, break; none can be found break; } Here, distance is a NumericVector, and ii and k are ints. My expectation is that this would dispatch to std::abs() with a double as its input. It's possible something is going wrong there. I'm wondering if this has to do with recent changes to R's C++ engine or compilers. If you want to run code to test whether the caliper is working correctly on your machine, you can run the following code: install.packages("MatchIt") data("lalonde", package = "MatchIt") m <- MatchIt::matchit(treat ~ age + educ + race + re74, data = lalonde, caliper = .01) summary(m)$nn If the caliper is working correctly, you should see a small matrix that has the row Matched 88 88 If not, you would see the row Matched 185 185 The GitHub issue of people complaining about this is here <https://github.com/kosukeimai/MatchIt/issues/163> along with their explanations about versions of *MatchIt* and R. The package code is also there. Any thoughts or insights about this would really help! Thank you so much! Noah [[alternative HTML version deleted]] ______________________________________________ R-package-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-package-devel
