While recession-squeezed companies are laying off full-time employes in record numbers, one segment of the labor force has remained relatively untouched--and may even benefit--in the slumping economy.

"The unemployment picture is turning out to be good for people who want to work part time," claims JoAnne Alter, author of a new guide to part-time work. Particularly in service and clerical fields, she says, "there are part-time jobs out there now that probably wouldn't be in better economic times.

"Companies that can't afford to hire full timers can often afford part timers--and some of those jobs may become full time later on. Many people who want full-time work are taking part-time jobs because that's all they can find.

"Over the last decade," she says, "the number of part-time workers has grown three times as fast as the ranks of full timers. Today, one out of every five people employed in this country has a part-time job."

Alter, 35, compiled "A Part-Time Career for a Full-Time You" (Houghton Mifflin, 390 pgs., $15.95) after favorable response to her articles on the subject in Family Circle magazine, where she is a contributing editor. She has held a variety of part-time jobs, including sales clerk, camp counselor, typist and teacher.

"A part-time job can be great or the pits," she admits, "depending largely on the marketable skills you have to offer. If you want to find a good part-time job, you've got to focus your job hunt on employers who really need the skills you have to offer and who are likely to be receptive to alternative work scheduling."

For example, "There's a desperate shortage of data processors right now, and anyone with those skills can probably get a job immediately at good pay." In an exceptional case, a major New York brokerage firm pays part-time word processors $30 an hour.

Exactly what constitutes "part-time work," says Alter, "depends on whom you ask. Someone putting in 35 hours will be considered a full timer at one company and a part timer at another." (The Labor Department says a part timer is anyone who works fewer than 35 hours per week by choice. Most part timers work between 15 and 30 hours per week, with a national average of 18 hours.)

Adult males make up the smallest proportion of part timers: about 16 percent. "There's still somewhat of a social stigma against men working part time," says Alter. "If a man wants to work part time, people wonder if he's really committed to the career game. Yet it's almost expected for a woman to want part-time work."

That stigma "is slowly changing," she says, as more men are recognizing that "there's more to life than climbing the corporate ladder." At certain times in men's lives, "when they're under 24 and over 65, they actually do seek part-time positions in greater numbers."

People work part time, says Alter, "for the same reason they work full time--money. In 1981, a family of four had to earn $22,477 before taxes to match the spending power that a $10,000 pre-tax income provided in 1970." The average female part-time worker is the mother of preschool or school-age children.

"Free time" and "flexibility," says Alter, are the two main attractions of part-time work. "You can work while you earn a degree, keep your job skills sharp while you enjoy your preschooler, maintain contact with coworkers while you relax and pursue leisure activities."

The flip-side: "Most of the negative aspects of part-time work center around money--or the lack thereof. Everyone knows that if you work less, odds are you earn less. Yet the discrepancy between full- and part-time earnings is more than just a matter ot the total number of hours worked.

"Once you account for differences in education and experience, race, family status and student status, male part timers earn (proportionately) 30 percent less than their full-time counterparts and female part timers earn 17 percent less. In 1979, part timers of both sexes earned, on the average, 29 percent less than full timers did."

One reason for the disparity: "Most of the nation's 23 million part-time workers are concentrated in lower-paying fields." The largest percentage of part-timers--nearly one-fourth--are service workers. About 23 percent hold clerical jobs, 13.3 percent are in professional or technical positions, 10.4 percent in sales, 5.9 percent laborers, 4.8 percent private household workers, 3.8 percent managers and administrators and 3.5 percent craft workers.

Another reason for low part-time pay, says Alter, is that "employers have traditionally been able to get away with (it) because many of the workers have been willing to trade flexible work schedules for low pay."

Other economic disadvantages:

"Only half of all part-timers get benefits. Many companies are not receptive to hiring part-time workers, and those who are often hire them only at the very lowest levels."

Since "loyalty is simply not expected of part-time workers," notes Alter, they often get passed over for promotions. "Yet, ironically, almost every study ever done on the subject has indicated that once part timers are hired, managers find they perform as well as, and often better than, full timers.

"Part-timers also exhibit more job satisfaction, less absenteeism and tardiness and lower turnover than full-time workers do. They tend to be highly motivated and perform at peak efficiency. I've heard managers say their part timers do as much in five hours as their full timers do in eight."

Because some employers may not see part-time help as the answer to their staffing problems, you may, says Alter, "have to really sing and dance to convince them."