I wrote the lead-author, David Figlio, of the piece that Ed and I have critically discussed. He responded promptly and helpfully to my questions (and piece). In particular, he emphasized that "by universal agreement, I was the only person to ever handle raw data, so Morty [Schapiro--ES] could never see individual student or faculty member names, or even departments." I quote the rest of his response in full:
We haven't yet controlled for class size, but I am certain, given the Northwestern circumstance, that what you propose won't be the driving force here. An initial view of the distribution of class sizes between long-term lecturers and tenure-track faculty showed that they have a high degree of overlap. We are in the process of trying a variety of additional sample splits; however, I can assure you that every specification we attempted yielded the same fundamental conclusion.
I am, by the way, pleased that you picked up on the fact that we are comparing long-term full-time lecturers versus tenure-track professors. At Northwestern, especially in the college of arts and sciences, where over 80 percent of the classes take place, has career ladders with four ranks; these lecturers have long-term contracts and the same benefits as tenure-track faculty members. Many accounts in the news media have missed this point, despite our repeated statements throughout the paper, and are trying to generalize our findings to one-off and journeyman lecturers. Others are trying to generalize to upper-division courses without basis. When reporters actually call me, I underscore these points, but they are often lost on the reporter. As is standard practice, we will make available the data after the paper has received its final acceptance at the journal.