Liberal arts colleges are four-year institutions that emphasize interactive instruction, although research is still a component of these institutions. They are usually residential colleges with most students living on campus in dorms. They tend to have smaller enrollments, class sizes, and lower student-teacher ratios than universities, and encourage teacher-student interaction with classes taught by full-time faculty members rather than graduate students known as teaching assistants. There are further distinctions within the category of liberal arts colleges: some are coeducational, women's colleges, or men's colleges. There are historically black colleges; in addition, while most schools are secular, some stress a particular religious orientation. Most are private colleges but there are some public ones.