If singers or their record compaines knew the pat answer for successful marketing, everyone would be a hit. But everyone isn't - so the experimenting continues.

There are a number of avenues open to popular music acts when "popular" describes their music and not the act. The Starland Vocal Band, at the Cellar Door next week, chose television. Dolly Parton stayed with records but added some commercial slickness.

Many people who have enjoyed success in the recording industry have successfully made the transition to television. Dean Martin, for one. Andy Williams for another. But after they became television stars a funny thing happened: Their records stopped selling.

Not completely, mind you. Martin and Williams are still good for some profits, especially around Christmas, but never near the returns for songs like "Everybody Loves Somebody Sometime" and "Moon River."

So seems the case with the more pop-oriented artists, most of whom have more to lose since their music reputations are not yet firmly established. First Paul Williams and then John Denver televised their record sales into near oblivion. Then, when it seemed that one hit record was the only requirement for a television program, acts like Sonny and Cher, the Captain and Tennille, Marily McCoo and Billy Davis Jr. and - finally - the Starland Vocal Band became television personalities at the expense of the musical support that got them there in the first place. (The Osmonds appeal to a much younger audience, so we can't really include them.) Cher now can't even give away her albums (before you snicker, remember that "I Got You Babe" and "Halfbreed," among others, were very popular) and the Captain and Tennille - once one of the biggest acts on the A&M Records roster - nearly dropped dead financially when they released an album after their show had been on the air awhile. (The album eventually did sell, but not as well as anticipated and only after much time and promotion.)

Washington's Starland Vocal Band got a Hollywood shot on the strength of their No. 1, "Afternoon Delight." Television seemed a great opportunity to broaden their appeal but, when placed in awkward television situations and forced to be glib and entertaining, the quartet proved lacking in mass-appeal glamor. Six shows later, they were gone from the screen. More significantly, a middle-of-the-road second album, released around the same time partly to cash in on their new recognition ("Rearview Mirror" RCA; BHL-1-2239), sold nowhere near what was expected. And now, a group that only last summer drew standing-room-only in a downpour at Wolftrap will play for considerably fewer people in considerably more shows.

Dolly Parton, on the other hand, never considered television - or television never considered her. Instead, the buxom country singer traded some of her cornpone for cotton candy and released "Here You Come Again" (RCA, APL-1-2544). There is still some countryish sentimentality evident in songs like "God's Coloring Book," but the title cut is pure pop with Parton's solid vocal fronting some hit-formula production. The result of all this is a smash single and her first gold album after years on the circuit.

What's more, Parton ironically has become a television punchline, a sort of Farrah Fawcett-Minors. Johnny Carson remarks on her cleavage at least once a week and Redd Foxx recently devoted an entire skit to the same topic. Despite the barely disguised sleaziness of most of the jokes, free publicity is free publicity and Parton burned up her last tour, selling out just about everywhere she played and winning over critic after critic. She may have lost a few hardcore country music fans, but she's gained immeasurably in every other area.

The conclusion, though not ironclad, is pretty clear. A musician's "availability factor" has a lot to do with that artist's success. The public won't usually buy what they can see and hear every week for nothing.