As a skeptic, I've watched the ascendancy of mix models for the last 20 years and I have been consistently puzzled by the ad industry's willingness to be measured by ad weight alone. Creative agencies, media buying groups, networks, copy researchers and advertisers all seemingly agreed to having their efforts evaluated by GRPs alone. This is because GRPs were and continue to be THE advertising input to mix models. In fairness, a few modelers have played around with other measures in the last few years, but for most of the last two decades GRPs alone were the sole model input for advertising.
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