Click-Through Rate Estimation for Rare Events in Online Advertising Xuerui Wang, Wei Li, Ying Cui, Ruofei (Bruce) Zhang, Jianchang Mao Yahoo! Labs, Silicon Valley United States ABSTRACT In online advertising campaigns, to measure purchase propensity, click-through rate (CTR), defined as a ratio of number of clicks to number of impressions, is one of the most informative metrics used in business ac