Randomization and The Pernicious Effects of Limited Budgets on Auction Experiments

Buyers (e.g., advertisers) often have limited financial and processing resources, and so their participation in auctions is throttled. Changes to auctions may affect bids or throttling and any change may affect what winners pay. This paper shows that if an A/B experiment affects only bids, then the observed treatment effect is unbiased when all the bidders in an auction are randomly assigned to A or B but it can be severely biased otherwise, even in the absence of throttling. Experiments that affect throttling algorithms can also be badly biased, but the bias can be substantially reduced if the budget for each advertiser in the experiment is allocated to separate pots for the A and B arms of the experiment.