When less is more : data and power in advertising experiments

Yahoo! Research partnered with a nationwide retailer to study the effects of online display advertising on both online and in-store purchases. We use a randomized field experiment on 3 million Yahoo! users who are also past customers of the retailer. We find statistically significant evidence that the retailer ads increase sales 3.6% relative to the control group. We show that control ads boost measurement precision by identifying and removing the half of in-campaign sales data that is unaffected by the ads. Less data gives us 31% more precision in our estimates — equivalent to increasing our sample to 5.3 million users. By contrast, we only improve precision by 5% when we include additional covariate data to reduce the residual variance in our experimental regression. The covariate-adjustment strategy disappoints despite exceptional consumer-level data including demographics, ad exposure levels, and two years' worth of past purchase history