Necessity, they say, is the mother of invention. So it's no surprise that, in today's rocky economy, the number of people who are launching their own businesses is growing.

Between 1972 and 1979 the number of self-employed Americans rose by more than 1.1 million, a trend that continued through the end of 1981, according to Labor Department statistics.

In prosperous times, says William Whiston, director of economic research for the Small Business Administration, "people make what they know how to make. But during periods of recession and high unemployment, people scratch around for new ideas. That's when your basement inventors and new-service providers get going."

This surge in self-employment occurs, says Whiston, "partly because someone who gets laid off has to find another way to make money. But it's also a psychological thing. Entrepreneurship is the American way. Making it on your own can be a measure of your personal ability to cope with a tough economic situation."

One of the greatest growth areas, says the SBA's Holly Medert, is "the small-service business. People today have less time, and new kinds of businesses are filling in the gap. There's a big increase in firms that do things like run errands and shop for busy people."

So, despite the grim statistics -- half of all new businesses fail in the first 38 months, eight out of nine don't make it past 10 years -- the pioneer spirit lives on.

Those who do succeed credit hard work, willingness to take risks and plain good luck. Among those who've taken the self-employment gamble and won are these three Washington entrepreneurs.

Ellen Wessel came to Washington in 1969 because she was "pretty psyched up on politics." But after working on several political campaigns, she discovered she had "no good sense of what the average American voter wants."

As a marathon runner, however, Wessel knew what the average American woman runner wanted.

Which is why, in 1977 at age 26, Wessel took her $2,000 savings, bought a $95 sewing machine and launched a company geared to manufacturing running clothes for women. Last year, Moving Comfort, Inc., netted $1.5 million in sales.

Wessel started running in 1974 to avoid gaining weight after she quit smoking -- a habit she picked up at age 14. Not surprisingly, the idea for Moving Comfort was born while she was doing her daily 10 miles with running partner Valerie Nye.

"Neither of us much liked the 9-to-5 work routine," says Wessel, who holds degrees in sociology and literature. "We were musing about businesses we could start . . . everything from a spaghetti restaurant to a bagel shop. Then we got the idea of making custom-designed running clothes for women.

"Both of us were tired of looking for running clothes that fit. They were all designed for men or were unisex at best. The biggest problem was chafing between the thighs, because the shorts were cut too short and skimpy at the hips and the fabric would bind in the crotch."

Wessel quit her job as a congressional liaison with HUD, Nye designed the first outfit and they found several seamstresses to sew the creations. As payment for an article she wrote for Running Times magazine, she was given ad space to announce the new venture.

"I got five letters from stores asking for our wholesale catalog. I wrote them all and said we would send them the new catalog when it came out. Then I had to go and find out what wholesale meant."

The fledgling company, located in her one-bedroom apartment, sold 50 pieces in the summer of 77. Then author Jim Fixx heard about Moving Comfort and asked if he could list them in the appendix of his "Complete Book of Running." Business boomed.

Today, three factories manufacture 400 dozen pieces of Moving Comfort items per week. Their Alexandria office employes 13 people and they have 40 sales representatives around the country. Wessel works 50 to 60 hours per week, often spending afternoons in the running clothes (Moving Comfort, of course) she dons for her midday 5-mile run.

Her advice to prospective entrepreneurs: "If you want to come up with a new product, don't worry about what other people need. Look at what would make your own life more comfortable, that ought to be there, but isn't. Chances are, other people will need it too."