A few years ago Paul Hertenstein of Golden, Colo., a draftsman by trade, enrolled in a radio and television repair course through the National Radio Institute, a private correspondence school.
Since completing the course, he has been sharpening his skills (and getting paid for it) by doing pick-up jobs in the evenings and on weekends in the shop of his basement. ''I will retire in three or four years,'' he said, ''and I plan to do this part time as an extra income.''
After graduating from high school a few years ago, John Kabot went to work in the dish room of St. Luke's Memorial Hospital in Utica, N.Y. He thought at the time that he would work there for a year or so and then go on to college and another career.
Today, thanks to his own hard work and a correspondence study program taken through the Pennsylvania State University, Mr. Kabot is the hospital's food service supervisor and has received offers of other management positions there.
Although the mention of correspondence study may still elicit sneers and derisive comments in some circles, the truth is that the quality of education through correspondence is high.
''We do not tell people that we are going to get them a job,'' said Michael P. Lambert, assistant director of the National Home Study Council, the nonprofit accrediting agency for correspondence schools. ''That is not our approach at all. We do tell them, however, that we can teach them a skill.''
Of the approximately 500 topics taught through National Home Study Council schools, 80 percent are in vocational, income-producing subjects like electronics, small engine repair, and accounting.
''Our typical student,'' Mr. Lambert said, ''is male, from 26 to 34 years old , and the head of a household. These people are looking for quality instruction in 'bench work,' moonlight skills, and that's what we have to deliver - quality. About 99 percent of them pay for the courses out of their own pockets, and we wouldn't stay in business for very long if we didn't give them what they had paid for.''
While the privately owned correspondence schools have always catered to students seeking vocational training, the university community is providing some innovations.
The University of Wisconsin-Extension offers ''continuing education units,'' a nationally recognized measure of classroom participation in continuing education programs, for work completed in a series of courses in air conditioning, bookkeeping, accounting, management, surveying, and a number of other courses.
''Most of these courses are at the professional level,'' said Dr. Donald F. Kaiser, director of independent study at Wisconsin-Extension. ''Many of the people taking our continuing education correspondence courses are already involved in the field. Our material permits them to grow.''
Of the approximately 10,000 enrollments in Wisconsin-Extension's independent study last year, about 4,000 were in continuing education correspondence courses.
A more recent addition to the University of Wisconsin-Extension program is the Wisconsin Vocational, Technical, and Adult Education System, a potpourri of courses ranging from ''Fundamentals of Interior Design'' to ''Technical Report Writing.''
The program provides a means by which students can explore their interests. ''We wanted to give people something to get started,'' said Dr. Kaiser. ''We wanted to help them decide what they want to invest more of themselves in.''
Graduates of Penn State's associate degree program in housing and food service are known as dietetic technicians.
They are taught to plan menus; interview, select, and schedule employees; supervise food production and service; maintain cost control records; participate with the dietitian in developing policies and procedures in the dietary department; collect diet histories among patients; and instruct patients and employees in various aspects of sound nutrition.
''Each student uses his or her institution as a laboratory,'' said Dr. David Mercer, director of independent study by correspondence at Penn State. ''The program is unique in this way, in the way the student is being taught.''
''One of the things we try very hard to do here in our office,'' says Prof. Ellen Barbrow, who functions as the program's coordinator in Penn State's College of Human Development, ''is to establish personal relationships with as many of our students as possible. They can call us any time; we sometimes even end up teaching over the phone.''
Teaching over the phone, of course, is not a rare thing in correspondence study. Donald Frank of Newark, Del., who took a radio and television repair course through National Radio Institute, said that his instructors ''were great.'' He felt he could call them any time with a problem or question, and that is just what he did.
Malcolm Massey, US Navy, retired, is a graduate of numerous armed forces correspondence courses and, like Mr. Kabot, used them to further his career.
''Correspondence study is a fine way of expanding your knowledge,'' he said. ''I've taken courses in damage control, communications, fire control, basic and advanced ballistics, international law, and personnel management. The list goes on. They helped me a great deal in my career,'' he said.
''The key today,'' National Home Study Council's Michael Lambert said, ''is to offer unique, relevant subjects. Our schools have no intention of supplanting resident instruction. It just won't happen. But what we can do is to offer new subjects, relevant for today. We can satisfy given needs in very specific areas.''
David Mercer at Penn State's Independent Study by Correspondence agrees: ''Fitting our educational program to the circumstances of the clientele is what correspondence study is all about.''