In 1975, 45% of teaching staff was tenure or tenure-track, 34% non-tenure-track (full or part time), and 21% grad students. In 2011, it's 24% tenure or tenure track, 57% non-tenure-track, and 19% grad students.
Now, part-time employees may make up a smaller proportion of actual teaching time than full time, but remember also that tenure track professors and grad students spend time doing research as well, not all of their time is spent teaching.
How many of your professors did you know the status of? I know that that certainly wasn't something I kept close tabs on when I was in college; there were a few professors who I knew well enough to know their status as tenured, tenure track, or not tenure track, but for many I didn't pay attention. Many of the adjunct professors are likely teaching intro-level classes that everyone is required to take; your freshman English or writing or math requirements.
Also recall that the more prestigious the school, the less likelihood that they will rely heavily on adjunct professors. Smaller state schools, community colleges, and for-profit schools may use them more often. Edit to add: the AAUP has another report that breaks down the faculty by school, and you can clearly see that these kind of non-tenure track and part time professorships are much more common at community colleges, smaller less prestigious schools, and for-profit schools, with some of them having 100% contingent (non-tenure track) faculty: http://www.aaup.org/sites/default/files/files/AAUPContingent...
And further, they may be named different things, but be roughly equivalent in job security, prestige, and benefits. Lecturer, instructor, visiting professor, visiting lecturer, assistant or associate teaching professor, etc.
Nonetheless, the AAUP document seems to show that even at US public doctoral and research universities, in 2005 non-tenure-track employees made up about a quarter of the full-time faculty and about 40% of all faculty if you include the part-timers (p. 18). There's also the issue of graduate students who teach or do research work. The numbers apparently included teaching grad. students, but only those which had been self-reported by their institutions as "employees" rather than recipients of valuable teaching/research experience (p. 10): it's not hard to imagine some under-reporting there.
In 1975, 45% of teaching staff was tenure or tenure-track, 34% non-tenure-track (full or part time), and 21% grad students. In 2011, it's 24% tenure or tenure track, 57% non-tenure-track, and 19% grad students.
Now, part-time employees may make up a smaller proportion of actual teaching time than full time, but remember also that tenure track professors and grad students spend time doing research as well, not all of their time is spent teaching.
How many of your professors did you know the status of? I know that that certainly wasn't something I kept close tabs on when I was in college; there were a few professors who I knew well enough to know their status as tenured, tenure track, or not tenure track, but for many I didn't pay attention. Many of the adjunct professors are likely teaching intro-level classes that everyone is required to take; your freshman English or writing or math requirements.
Also recall that the more prestigious the school, the less likelihood that they will rely heavily on adjunct professors. Smaller state schools, community colleges, and for-profit schools may use them more often. Edit to add: the AAUP has another report that breaks down the faculty by school, and you can clearly see that these kind of non-tenure track and part time professorships are much more common at community colleges, smaller less prestigious schools, and for-profit schools, with some of them having 100% contingent (non-tenure track) faculty: http://www.aaup.org/sites/default/files/files/AAUPContingent...
And further, they may be named different things, but be roughly equivalent in job security, prestige, and benefits. Lecturer, instructor, visiting professor, visiting lecturer, assistant or associate teaching professor, etc.