Overexposure of individual ads was a frequent concern when Mark Addicks joined General Mills Inc. in 1988 and television dominated the food company's advertising. These days, though, the Internet lets him target loyal customers with messages related to General Mills brands without offending casual buyers with repetitive ads.

"You can use the same piece of creative work in so many different ways," says Mr. Addicks, now General Mills's chief marketing officer. When he sought maximum impact from a Cheerios ad showing two newly adopted children and their new parents, he aired one version on television, a longer version in movie theaters and a third version on the Internet. That variety helped General Mills reach a bigger, more diverse audience without taxing viewers' patience, he says.

Advertising wear-out has fascinated researchers since the 1970s. Studies, generally focusing on television commercials, have suggested ads can be viewed anywhere from three to 25 times before losing effectiveness. After that, says Brian Sternthal, a marketing professor at Northwestern University's Kellogg School of Management, people "dismiss the information as old news."

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