Until recently, believing in the effectiveness of advertising and promotion has largely been a matter of faith. Marketing departments might collect voluminous statistics on television program ratings and on coupon redemptions and carefully compare the costs of marketing with total sales. But none of this data measures what is really important: the incremental sales of a product over and above those that would have happened without the advertising or promotion.
A version of this article appeared in the May–June 1990 issue of Harvard Business Review.