EAU CLAIRE, WIS. -- Fifty-five thousand souls. A tire factory, a paper mill. Near the city's core, restored mansions sit next to dilapidated ones cut up into apartments; on the outskirts of town, tidy houses line the slow curves of the suburban roads. There's a downtown that's about as lively as a whipped dog, and long, treeless streets crammed with franchise outfits and strip malls. A glittering mall sits on the edge of town; inside, it looks like an embassy for a nation far more fascinating than Eau Claire ever could be.

There's nothing much here you can't see in a thousand other towns its size across the country. Which might be why a Chicago-based marketing research company called Information Resources chose this town for its project.

Information Resources, a company devoted to finding out who buys what and why, has been testing products for eight years. Eau Claire is one of the six cities it uses to mirror the rest of the nation in an overall operation called Behavior Scan.

Information Resources -- Shoppers' Hotline to the consumers -- uses cable TV to send commercials for certain products to a selected group of households. It puts ads for those products in the households' newspapers. When those people go to shop, the checkout scanners register whether they've bought the advertised product.

And the computers tell some of the nation's biggest food manufacturers -- Procter & Gamble, Del Monte, Frito-Lay, Heinz, Kraft, Nestle, Nabisco -- the age, income, sex and educational status of the purchaser. Based on this data, a company can scotch a product or give it a national send-off.

And the 3,500 volunteers who belong to the Eau Claire Shoppers' Hotline like the idea of cheering on a new product or giving it a big, wet raspberry. It's up to them to save the nation from Oat Bran Oreos or Nacho Pickles, and they do not shirk from the duty.

Take Natalie Steinke, a Shoppers' Hotline member. Does she feel that this program gives her a sense of power?

She laughs. "Power? Oh, I don't know." She thinks a second, then smiles. "Yes."

But why Eau Claire? According to Bob Briginzer, senior vice president of corporate communications for Information Resources, it comes down to the size of a town, not whether everyone who lives there looks like they're auditioning for "Father Knows Best."

Eau Claire's modest proportions mean that a sample of 3,500 people is representative of the rest of the town. The same is true of the other Behavior Scan towns: Cedar Rapids, Iowa; Midland, Tex.; Marion, Ind.; Grand Junction, Colo., and Pittsfield, Mass.

The program also requires that a local paper carry the special ads and that a high percentage of homes are hooked to cable, so the special commercials can be sent to homes without having to wire them up. Local merchants need to agree to carry the new products and collect the data from the supermarket scanners.

Eau Claire qualifies.

"No city's perfect," Briginzer says, "but Eau Claire is close."

But surely acquiescent merchants and a strong local media are not enough. Surely there is something innately typical about this town and the people who live here.

"Typical?" says Natalie Steinke. "We're not typical. We're older. We don't have two-point-five kids. We don't have a dog or a cat, and we don't live in a house."

Natalie and her husband, LaMoine, Shoppers' Hotline members for nearly eight years, are in their mid-fifties, semi-retired (they share a job at a property-management company), and their kids have grown up and left home.

Her years as a hot line member have caused Natalie to look at commercials differently. "There are a lot of things I suspect," she says. "Miracle Whip with reduced cholesterol? I wonder. Cereals with bran? I wonder. Anything that seems to have older actors and is aimed at older people, I suspect."

Do her suspicions affect the way she shops?

"Well, I know what the ads are supposed to do; they're supposed to make me want the product. But does an ad make me buy something I don't want to buy? No."

To demonstrate this, the Steinkes go to Randall's grocery store in a mall-infested part of town. It's a thoroughly modern supermarket with every amenity: Video rental. A liquor store. A salad bar. A cash machine. ("Free peanut-butter dip, too," LaMoine points out.) There's also a rack of Shoppers' Hotline applications. Like all other grocery stores and most of the drugstores in town, Randall's is hooked up to the hot line. If Natalie buys something that is a test product, the folks in Chicago will know it.

They haven't gone 15 feet before they encounter Hot Wraps. Bagel dough stuffed with sausages. A woman in an apron stands behind a small stove, skewering Hot Wraps with toothpicks and putting them out for all to sample. Natalie tries one and says it's not bad, "but not the kind of thing we'd go for."

Why? "Expensive, and, well ... " Her expression fills in the rest: sausages wrapped in bagel dough.

Around the corner is a display for New Cuisine Cubes. Natalie hasn't seen ads for this, but even if she had, nothing could make her buy it.

"First of all, I don't know what it is." She holds up the brightly colored package, studying it. "Looks like it's made of flowers. Okay, it's frozen seasoning concentrates. Well, I can make my own." On to the next aisle.

The microwave popcorn prompts another discussion: "Look at this. Says 'lite.' I am not particularly fooled by 'lite.' A pretty useless term. You have to investigate it. This stuff, for example, has all kinds of oils in it, stuff that's awful for your cholesterol."

LaMoine has disappeared. Natalie thinks he is off sampling the free peanut butter.

In the next few aisles are flavored milk in single-serving packets, canned macaroni fashioned in shapes designed to appeal to kids, new cookies stuffed with fruit. She notes their appearance, comments on the wasteful packaging, the price, the amusing commercial she saw.

"Now," she says, "I'm going to buy something I suspect. Rice bran cereal. I saw a commercial for this, and it looks like something we could use."

The rest of the shopping excursion is uneventful; only Microwave Zap-a-Pack Cheez Whiz earns a "highly suspicious" from Natalie. She meets up with LaMoine at the peanut-butter bar. It's a house brand and remarkably good. Packaged in a plain plastic tub. Natalie buys some.

"Shoppers' Hotline?" says the checkout clerk. Natalie produces her card.

Does the clerk always ask?

"I have to ask everyone," says Eric, the clerk. "And 50 percent of the customers have one."

The Steinkes had just been through the sort of shopping trip millions of others make every day. But when Natalie handed over that card, the scanners sat up and took notice of her purchases. If that rice bran was indeed a product being test-marketed, her purchase might have been the one to push the numbers into the category marked "Go."

That box of cereal might sit uneaten on the Steinkes' shelf for weeks, or it might be emptied in a week. It doesn't matter. What counts is that someone said "yes" to rice bran. And if the Steinkes said yes, so will millions of other Americans.

"We have a waiting list," says Sue Norgaard, assistant manager for Behavior Scan in Eau Claire. "Our clients dictate what kind of people we need, and as they need new clients, we add them."

There is no direct financial incentive to join; members do not get discounts on purchases. But there are contests. "It's like a lottery," Norgaard says. "The Shoppers' Hotline members, as an incentive to show their cards at the registers, get chances to win prizes. The top monthly prize is $1,000 or a trip; the fourth prize is $10."

Most people win only $10 -- Natalie Steinke did once. No one she knows ever won a trip, however.

"Compensation is tricky," Information Resources' Bob Briginzer says. "You want to do something for them, but you can't pay them a great deal, or you feel like they've changed what they would otherwise do, and they feel they owe you something.

"They don't know they're looking at new products. Maybe they do, but they're paying their own money for it. So, if they don't like it, they won't try it again. The thrill of being in this program wears off, and you tend to forget that you're doing anything special."

But what exactly is being tested here? Natalie Steinke bought the rice bran cereal because she thought it was a useful product. Stories in the media had told her bran was good, and the commercials, capitalizing on the public's awareness of this miracle food, had shown her a way to get it. Is the Shoppers' Hotline a means of gauging whether a product is liked -- or whether the marketing works? Says Briginzer:

"As to whether advertising or packaging can persuade someone to buy something they don't need or don't want, I don't think it can. But if they need cold cereal, ads can help them make a decision on oat bran versus presweetened cereal. It provides information, and we record how the consumer reacts when given that information."

So after the graphic designers have created the perfect package, after the marketing strategies have been laid out, the spokespeople chosen and the dazzling commercials shot, it all comes down to someone standing in a supermarket in a medium-sized city, cart groaning with items, saying, "Do I really need microwaveable Cheez Whiz?"