IF YOU WATCH many TV commercials, you've probably asked yourself more than once: "What next?" One answer was contained in a story in this paper last week on another advance in advertising and market research -- the "RD-100 People Meter and Data Scan Wand." T. R. Reid's report from Denver gave the particulars of this breakthrough:

Under a program devised by Arbitron, the television rating firm, and Burke Marketing Services, the TV sets of volunteer families will be fitted with equipment to monitor both their watching habits and their buying patterns. The "People Meter" takes control of the screen every 30 minutes to ask who is watching. Before viewers can see the next show, one of them must punch in on a keypad the age and sex of everyone in the room. Attached to the meter is the "Data Scan Wand." The volunteers, after they go shopping, are expected to run the wand over the universal product code of everything they buy. Thus is Arbitron's computer in New York provided with the link, if there is any, between commercials viewed and products purchased.

"We know people will volunteer for this," said Burke vice president Robert L. McCann Jr. "Pavlov knew what the dog would do when the bell rang, and we know what people will do."

We know what people will do, too. They will soon tire of hurrying to the TV set to register all their groceries with the Data Scan Wand before the frozen yogurt melts, and they will assign the task to their children. The children will register only the things they like -- several times. The market researchers will spot a trend toward caramel-covered popcorn, bubble gum, creme-filled cupcakes, grape soda and sugar- coated Jujubes. All of these products will be heavily advertised, displacing commercials for beer and GT cars. Viewers will scratch their heads. Forward-looking manufacturers will form a PAC to work for an end to the embargo on Cuban sugar and contribute $35 million to members of Congress. The chairman of the Senate Agriculture Committee will see this as evidence of a Cuban-Soviet plot to subvert the federal tobacco program, and will call hearings at which testimony will be heard from hooded witnesses. They will be asked point-blank whether they registered all the groceries as their parents told them to, and will give evasive answers. Rare Saturday sessions of the House and Senate will be held to pass a bill placing the marketing research business under the purview of whatever federal regulatory agency is left. The economy, which had begun to list heavily toward sweets, will right itself, but only after a number of fortunes are made and lost on Wall Street.

All of this doesn't have to happen, but the prospect of it is good reason to watch what you do with that wand if they give you one.