Consumer Heterogeneity and Paid Search Effectiveness: A Large Scale Field Experiment
by Thomas Blake, Chris Nosko, Steven Tadelis · NBER / Econometrica
eBay ran large-scale randomized experiments switching off paid search ads across matched markets. Returns to branded keyword advertising were near zero because those clicks cannibalized free organic traffic, and non-experimental estimates overstated effectiveness by a wide margin. The single most consequential empirical result in paid search, and the reason it survives despite the pre-2022 caution: this is not a tactical guide tied to a UI, it is a causal finding about the economics of search advertising that has not been overturned.
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