Several studies of individual universities have determined that freshmen taught by many part-timers were more likely to drop out.

“Having an adjunct in a course is not necessarily bad for you, but having too many adjuncts might be,” said Eric P. Bettinger, an economics professor at Case Western Reserve University in Cleveland.

Students say they can often tell when a professor is part-time. Mike Brennan, a sophomore at the University of Michigan, Dearborn, said the courses taught by adjuncts tend to be more basic and the exams less challenging. “They have so many classes that they give tests that are easier to grade,” Mr. Brennan said.

Carly Matkovich, a senior at the university, said she had bonded more with her part-time teachers, in part because they have more practical experience. But it is usually hard to find time to talk with them outside class. “They’re never around,” Ms. Matkovich said. “It does make me feel kind of cheated.”

At some departments the proportion of faculty who are tenured is startlingly low. The psychology department at Florida International University in Miami has 2,400 undergraduate majors but only 19 tenured or tenure-track professors who teach, according to a department self-assessment. It is possible for a psychology major to graduate without taking a course with a full-time faculty member.

“We’re at a point where it is extreme,” said Suzanna Rose, a psychology professor who said she stepped down as department head in August, primarily because she could not hire as many tenure-track professors as she thought the department needed. “I’m just very concerned about the quality.”

Ronald Berkman, the provost at Florida International, disputed her numbers, saying the psychology department has 23 professors who are tenured or tenure track and 5 full-time teachers on contracts. The department is conducting a search for three more tenure-track professors, Dr. Berkman said.