While the study did not deal with part-time adjuncts hired semester to semester, David N. Figlio, the lead author, said in an interview that it had provided evidence that what mattered for students was full-time teaching status, not tenure.

“The rise of full-time designated teachers at U.S. colleges and universities may be less of a cause for alarm than some people think,” the study said.

But a preliminary national study, presented recently at a higher education conference, found that the percentage of part-time faculty members at community colleges had no impact on student success.

Organizers from the service employees’ union, analyzing local living costs and adjunct pay, said at the Boston symposium that while teaching 12 courses a year was “an extraordinary course load,” the average adjunct would have to teach 17 to 24 classes a year just to afford a two-bedroom apartment and utilities — then another two to four classes to cover groceries. Saving for retirement and going out for meals was out of the question.

The meager reach of adjuncts’ pay was no surprise to William and Barbara Shimer, who are adjunct professors at Northeastern. In all, they are teaching 11 classes this semester — he has five at Northeastern and two at Wentworth, she has four at Northeastern — paying $2,100 to $6,500 each, depending on where and for how many credits.

Mr. Shimer was once asked on a Friday to teach a new course starting the following Monday, and told to come early that day to pick up the textbook. Ms. Shimer once had a class she was counting on canceled shortly before the first class. Like most part-time instructors, they have neither job security nor health benefits. But what pulled Mr. Shimer into the union campaign was the lack of an office.

“We used the trunk of our car as our office, rushing back between classes to dump one set of books and materials and get what we needed for the next class,” he said. “Then one day, our office got towed. I decided right then to get involved with the union.”