Do movie ratings work? Of how much value are they to their target audience - parents? And how accurate are our perceptions of what a G, PG, PG-13 or R-rated movie will contain, in terms of potentially offensive content?

Those are tough questions, and on the 20th anniversary this week of the inception of the Classification and Ratings Administration, a subsidiary of the Motion Picture Association of America, it seems appropriate to assess two decades of rating some 8,500 movies.The stated purpose of the ratings board is to help parents know which movies they might wish to steer their children toward or away from.

The reality is that many people use ratings as a tentative barometer to determine whether they or their children should seem a film.

But the ratings themselves are rather vague.

The two extremes - G and X - are virtually extinct. With rare exceptions, G-rated movies are innocuous animated features for children. Major studios will not release X-rated movies, requiring that they be edited, if necessary, to get an R. (Pornographic movies carry self-imposed X ratings and are not rated by the ratings board. The X is the only ratings symbol not registered by the board.)

That leaves PG, PG-13, and R.

While most parents may feel the R rating automatically means a film will be offensive, they tend not to realize that a PG or PG-13 rated movie may contain material that is equally offensive.

Nudity, excessive profanity and vulgarity, gory violence and sex scenes are as common in PG and PG-13 movies as they are in R-rated films, though the latter may carry those elements to further extremes.

But even the R rating can be deceptive when an inoffensive, family-oriented film like "The Milagro Beanfield War" carries the same rating as "A Nightmare on Elm Street 4."

Despite these inequities, the movie rating system is not without value.

Using the ratings as general information and in tandem with critics' reviews, word-of-mouth from friends, and publicity can help moviegoers more intelligently determine which films will entertain and which will offend.

Jack Valenti, MPAA president, says 65 to 73 percent of 2,600 parents recently surveyed found the ratings "very useful to fairly useful."

But they should be taken with a grain of salt, and certainly should not be the decisive factor in determining whether a movie is suitable for children.