Harvard Extension School enrollment rose by more than seven percent last year, topping off an almost 100 percent increase in its number of students since 1975, according to a recent report.

The Extension School, an open admissions program which offers evening liberal arts courses to part-time students, reached a peak enrollment of 11,170 last year, up from 6206 in 1975, and University officials say they expect further increases this year.

Although the program does not require students to have college degrees, the recent expansion represents an increase mainly in the number of post-graduate students, who currently make up 70 percent of the Extension student population, the report states.

Michael Shinagel, dean of Continuing Education and University Extension, attributed the rise to University efforts to improve and expand the school's curriculum and degree programs, as well as growing national demand for continuing adult education.

"There have been steady increases for quite a while in the number of people who want to pursue continuing education courses," he said, adding, "Only during the last eight years, though, I think, have we expanded our programs to fill the demand."

Last year, the Extension School offered 435 courses, compared to 398 two years ago and 194 in 1975.

Although the program draws its faculty from Harvard as well as several institutions which co-founded the first Extension courses in 1910, the University has increased the proportion of courses taught by Harvard faculty as the curriculum has expanded.

Currently, University faculty members teach 70 percent of the courses, and instructors at other local schools make up the remainder.

The University has enlarged its offering of professional and technical courses and, in 1980, established a Masters in Extension Studies degree program.

More than 800 students are now taking the school's introductory accounting course or the introductory computer course, and 1050 students received credit in Master's degree courses last year.

Robert Ross, president of the Extension School's student government, said yesterday the greater popularity of Extension courses "comes down to a question of cost-benefit analysis."

"Harvard's extension school is less expensive than any in the area and students are now able to clearly advance their careers by taking these course," he explained.

The University's incentive to expand the program is partially economic as well, Shinagel said.

"We are talking about a clear example of the 'economics of unused capacity'," he said. "Extension classes are taught in regular university buildings which stay open whether or not evening classes are scheduled." The additional physical overhead costs to the University are close to zero, he added.

Referring to the school's Bachelor of Arts program, Ross said, "You could go for four years to the Extension School, spend $4000 to $5000, and get into Harvard Medical School, or you could go to Harvard College, spend $60,000, and get into Harvard Medical School."