Picking cover subjects is an inexact science—but the study of celebrities doesn't have to be. Witness PEOPLE-Yankelovich Pop Monitor Profiles, a new marketing service that determines how various demographic groups rate America's icons in terms of such qualities as likability and trustworthiness. Based on a semiannual study conducted by PEOPLE marketing and Yankelovich Partners Inc., the Pop Monitor is now being offered by PEOPLE'S publisher to advertisers and the entertainment industry as a way to assess how celebrities reflect or influence cultural trends. For instance, Sally Field, who rates very highly overall, represents a recent move toward valuing home and hearth over status and flash. The idea behind this sophisticated marketing tool, says PEOPLE publisher Nora McAniff, will be to help the media industry "to better develop programming, spot emerging stars and identify spokespeople. Pop culture is full of change and energy, and understanding the celebrities who drive it economically can be essential to any marketing strategy."