IT'S NOT a common event to celebrate the tenth anniversary of an idea that worked, but that is what the Motion Picture Association of America has been doing, and we join them. The idea is the movie-rating system. Jack Valenti, the president of the MPAA, thought it up at a time when film-makers were operating under few restraints, and individual cities and states were seeking to establish local codes. In April 1968, the Supreme Court upheld their constitutional power to do so. Serious film-makers were in danger. We could have looked forward to hundreds of separate censorship boards, and a national norm in which "Son of Flubber" would have been considered risque.

Before that time, movie studios had operated according to the rigid standards of the Production Code Administration, whose lines of propriety had been enforced since the 1930s. By 1968, those lines were either going to be drawn tighter or broken through, and if they were to be broken through for a while, they eventually were bound to be drawn tighter still. Mr. Valenti's idea avoided both consequences. It made it possible to produce a healthy variety of moves, ranging from "A Clockwork Orange" to "Heaven Can Wait," while enabling parents to anticipate the difference between "The Shaggy D.A." and "Fritz the Cat."

Naturally, the rating system hasn't been perfect. The X rating, for example, has almost exclusively produced junk, thereby falseby suggesting the incompatibility of explicit sexuality and seriousness. And there have been reasonable complaints about the ratings of individual pictures, along with the raising of such questions as whether violence is more offensive than sex, and so forth. On the whole, however, the rating system has allowed the more imaginative film-makers to do their best, and has encourage a sense of public responsibility on the part of the movie industry generally. Any idea that can do that can't be all bad.