Pioneering researcher Thomas Ebbinghaus did a study on people's ability to retain information. He called the results the "Curve of Forgetfulness."

Ebbinghaus found that a person forgets 75% of what he or she has learned in the previous week. After three weeks, he/she forgets 90%. After four weeks, he/she forgets 95%.

The Ebbinghaus study is an illustration of retentiveness; the brain retains information it considers important to the individual and "forgets" information not deemed relevant.

In terms of advertising, it is crucial for the advertiser to make the message important enough for the individual to want to retain it and act on it.

—The Arbitron Company, Beyond The Ratings