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Measuring the causal effects of online advertising (adfx) on user behavior is important to the health of the WWW publishing industry. In this paper, using three controlled experiments, we show that observational data frequently lead to incorrect estimates of adfx. The reason, which we label "activity bias," comes from the surprising amount of time-based correlation between the myriad activities that users undertake online. In Experiment 1, users who are exposed to an ad on a given day are muchdoi:10.1145/1963405.1963431 dblp:conf/www/LewisRR11 fatcat:gyasxdrgwjhflfwcij4g72jhqy