In R, when I used clogit, I encountered the error Warning message: In fitter(X, Y, istrat, offset, init, control, weights = weights, : Ran out of iterations and did not converge

Upon some “Google fu”, I found this page, which says “Without checking it out thoroughly, my guess is you have Complete Separation (or close to it). What that would mean is that there aren’t enough retr=0 that match up with case=1 (or some other combination). R can’t calculate the odds of success if there are no successes in one of your conditions. The odds become infinite.”