At a time when colleges are under increasing financial pressure to rely more on part-time instructors, three new studies suggest that doing so erodes the quality of education many students receive. Part-timers' inability or unwillingness to devote more time to students outside the classroom, the research suggests, results in the denial of important support services to many students--including, often, those who need the most help. In a finding that breaks new ground, one of the studies concludes that heavy reliance on part-timers can actually hurt the performance of full-time, tenure-track faculty members. One possible explanation: Full-timers feel less secure at institutions that might replace them with part-time instructors and lecturers, said that study's author, Paul D. Umbach, an associate professor of adult and higher education at North Carolina State University. Two of the studies, focused on the academic success of community-college students with part-time instructors, are by Audrey J. Jaeger, an associate professor of higher education at North Carolina State University, and M. Kevin Eagan Jr., a graduate student at the University of California at Los Angeles.