To buy, or not to buy -- how do we decide?

Why do we choose one brand over another?

Marketing experts are convinced that advertising plays a key role. Others would tend to agree. But just how large a role and which advertising techniques are most effective are topics of vigorous debate. This controversy is one reason advertising methods tend to vary widely and recur in cycles over the years. An estimated $62 billion was spent on advertising in 1982, but the search for what motivates consumers to buy has not ended.

''There are lots of reasons'' that people buy products - reasons ''that they'll never admit,'' says Allen Smith, associate professor of marketing at Florida Atlantic University. ''They'll say they bought a Mercedes because of its quality or gas mileage - they may not want to tell you they bought it because it has snob appeal. . . . The bottom line is that advertising is a very hard subject to study. Everyone has his own opinion of what works best.''

Some university marketing experts say the current recession has spawned a resurgence of ads offering special discounts, coupons, and contests. They point to the recent flurry of games and prizes offered by fast-food chains and to car-rental-agency giveaways of everything from TV sets to luggage.

''Advertising campaigns have changed dramatically over the last year or so - there are lots more of the 'buy one and get a coupon toward something' variety geared to the current economy,'' says Roger Blackwell, professor of consumer behavior at Ohio State University. In his view it's all part of a response to ''frontier consumerism,'' in which both spouses in most families work and the only remaining way to make income stretch is to shop for more value for each dollar spent.

''As people grow more careful,'' Dr. Blackwell explains, ''they look for advertising that is more explicit in pinpointing value and communicating useful information about a product.''

Coauthor of a leading college textbook in the advertising field, Dr. Blackwell says he also sees a move away from the kind of advertising that tries to build a favorable attitude about a company or product, such as ''Fly the Friendly Skies of United'' or ads extolling the environmental care taken by certain oil firms. Instead, he says, many ads now try to get the consumer to change his behavior and develop brand loyalty by offering a reward. One current airline ad campaign, which is aimed at the frequent flyer and offers free trips as a bonus for miles logged, is a classic case of what he calls the ''B-mod'' (behavior modification) approach.

Most marketing experts say they think the current emphasis on comparative ads , which tout the merits of one brand over others, will continue. Officially encouraged by the Federal Trade Commission since 1979, the technique is practiced by promoters of everything from hamburgers to peanut butter. Some campaigns, such as the Pepsi-Cola challenge to compare the Pepsi taste to other colas, have been notably successful. But many experts still question the general effectiveness of such ads.

''Some parents don't like the implications for children of name-calling and criticizing other products instead of naming the good things about yours,'' Professor Smith says. ''If what you say is true, and people find it so, this kind of advertising can be effective. But sometimes people don't pay close attention and think the product being disparaged is the one doing the advertising.''

Indeed, this worry that the consumer might remember only the competition is one reason some advertisers soft-pedal the naming of rivals. A current ad-coupon for Van de Kamp's fish sticks, for instance, carries the headline: ''It'll Make You Forget Mrs. What's Her Name.''

''Any ad that sends people away without remembering their product is a bad ad ,'' says J. Steven Kelly, associate professor of marketing at De Paul University.

One hard sell, the 15-year-old ad campaign that many consumers and ad experts label boring and obnoxious - but a definite winner when it comes to getting consumers to remember the product - is Wisk detergent's pledge to remove ''ring around the collar.''

It's one of the most disliked commercials, Professor Kelly says. ''But if you ask the man on the street what detergent eliminates that problem, four out of five people will probably say, 'Wisk,' '' he adds.

While consumers may enjoy humorous ads as entertainment, there is very limited evidence that they are more prone to buy the product advertised.

''The negative thing about humor is that it takes away from the time you mention your product and that consumers don't necessarily remember it,'' says Joanne Cantor, associate professor of communications at the University of Wisconsin, who has run several tests on the short-run effectiveness of such ads. Among her findings: Children who watched a promotion in which an adult lectured on the dangers of eating sweets were more likely to spurn dessert at lunch than youngsters who saw a lively ad in which a group of oranges praised themselves as snack alternatives to sweets.

''There is little evidence that humor adds to persuasion, and some evidence that it distracts and may actually be detrimental,'' Dr. Cantor says.

Questions in slogans or jingles - such as McDonald's line a few years ago on what ingredients go into a ''Big Mac'' - are generally considered effective ways to get consumers involved in ads and help them remember the product.

''You don't put in question form something that's easy to refute,'' Dr. Cantor says. ''But, generally, if the consumer doesn't have a better answer, a question is more acceptable than a bold statement of fact.''

But even as the search goes on for more effective ad techniques, experts say future ads are sure to remain varied.

''With so much money being spent on advertising, it becomes a real challenge to come up with something different,'' says Robert Hartley, professor of marketing at Cleveland State University. ''But everyone is groping to come up with a unique selling proposition. If they all do the same thing, it loses its effectiveness.''

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