@Alexandre
Yes, the Student:Faculty ratio is a crude metric for comparing class sizes.
I’m skeptical that private universities consistently are less honest than public universities in calculating and reporting this figure … if so, has this story been picked up in the press or in any academic journal? … but there may indeed be too muich wiggle-room for different calculation methods. Fortunately, there are more precise ways to assess class sizes.
For several reasons, I believe that highly selective private universities (like NU) do tend to have significantly smaller classes than state universities (even Berkeley or Michigan) . First, it is indicated in the data reported in the Common Data Sets. The CDS, section J, not only shows a Student:Faculty ratio (in section I2), it also shows undergraduate class size distributions (in section I3). For example, NU reports 1466 undergraduate classes with less than 20 students, which is 75% of the total (1962 classes). NU reports 44 undergraduate classes with 100+ students, which is 2% of the total (1962 classes). In contrast, Berkeley reports 11915 undergraduate classes with less than 20 students, which is 60% of the total (3192 classes). Berkeley reports 270 undergraduate classes with 100+ students, which is 8% of the total (3192 classes), or ~4X NU’s percentage. Berkeley, by the way, seems to have among the the lowest average class sizes of any state research university. UC Davis or UCSD for example report much greater numbers of big classes. Then again, some private unis, such as Cornell, report bigger class sizes than NU (http://talk.collegeconfidential.com/college-search-selection/708190-avg-class-size-p1.html)
Second, there is the anecdotal evidence I see on College Confidential, or my own and relatives’ personal experiences at several kinds of institutions. Complaints about huge classes at state universities are fairly common. They seem to be less common for “elite” private universities. Have a look at some of the evaluations on studentreviews.com. On that site, Berkeley gets 32 negative reviews. Northwestern gets 25 negative reviews. I read through the first 18 of the negative Berkeley reviews; of these, I count 11 that complain about big classes. In the 24 negative NU reviews, I do not see a single complaint about big classes (although students complain about other things, such as snobby classmates or heavy workloads … and in a couple of reviews, about professors who don’t have enough time for students).
Third, I’ve compared introductory and intermediate course enrollment numbers reported in the online Fall/Spring course registration pages for several several selective private universities (e.g. Chicago, JHU, Princeton) , state universities (e.g. Berkeley, Michigan), and LACs (such as Williams). Rather than rehash what I’ve reported in other threads, I recommend the OP do this for himself, if he cares about class sizes. Focus on the classes that most interest you, because the overall class size averages may not be reflected consistently in all departments.
I do agree with Alexandre that you’ll also find big classes (>= 50 students, or even 100+ students) at private research universities. Those schools also do use TAs. You may decide that, whether a lecture class has 80 students or 400, a lecture is a lecture (although I think the size can affect office hour availability and other access). I agree with him that if you really want to avoid big classes and TAs, you need to choose a LAC (which for many students have other disadvantages).