It may have been something about the proportion of interviews compared to the apps you send out...I can't quite remember, but I'm searching through the old thread.
The numbers from the old thread, as docma mentioned, are actually available under the "match statistics" section(s) of APPIC's website.
Basically, they look at the average number of rankings submitted by individuals who matched (somewhere between 6 and 7 IIRC) vs. those who didn't (between 3 and 4, again, IIRC).
This obviously doesn't give you an exact idea of how many interviews each group received, although I would imagine it generally holds that most applicants will not rank most sites at which they didn't receive an interview. Keep in mind, though, that there's no "penalty" for ranking sites. Thus, I'm sure more than one applicant ranked sites they didn't interview at, just as I'm sure some applicants didn't rank sites where they did interview.
I'm not sure of any data that shows the average interview "return" rate (i.e., the average ratio of interviews to applications sent) for matched vs. unmatched applicants, though. Off the top of my head, and thus as a COMPLETE guess based on gut instinct and what I remember from last year, I'd say that if you're getting a ~40+% return on 15-20 applications, you're probably doing fairly well. Just as a personal example, another applicant from my program and I were both at a bit over 40% and both fortunately ended up matching.
Then again, as cliche as it might sound, always remember--it really does only take ONE great interview and fit. On the flip side, there are no guarantees regardless of return rate/number of interviews.