Trustworthy and Powerful Online Marketplace Experimentation with Budget-split Design

Online experimentation, also known as A/B testing, is the gold standard for measuring product impacts and making business decisions in the tech industry. The validity and utility of experiments, however, hinge on unbiasedness and sufficient power. In two-sided online marketplaces, both requirements are called into question. The Bernoulli randomized experiments are biased because treatment units interfere with control units through market competition and violate the "stable unit treatment value assumption"(SUTVA). The experimental power on at least one side of the market is often insufficient because of disparate sample sizes on the two sides. Despite the importance of online marketplaces to the online economy and the crucial role experimentation plays in product development, there lacks an effective and practical solution to the bias and low power problems in marketplace experimentation. In this paper we address this shortcoming by proposing the budget-split design, which is unbiased in any marketplace where buyers have a finite or infinite budget. We show that it is more powerful than all other unbiased designs in the literature. We then provide a generalizable system architecture for deploying this design to online marketplaces. Finally, we confirm the effectiveness of our proposal with empirical performance from experiments run in two real-world online marketplaces. We demonstrate how it achieves over 15x gain in experimental power and removes market competition induced bias, which can be up to 230% the treatment effect size.