This document discusses how click-through rate (CTR) is an imperfect metric for measuring ad effectiveness. While CTR averages 0.3%, only a small segment of "heavy clickers" accounting for 6% of internet users generate around half of all clicks. Heavy clickers have different online behaviors and intentions than most consumers. The document recommends measuring outcomes like conversions instead of clicks, and using behavioral targeting to better understand consumer intentions behind clicks. Relying only on CTR can provide a misleading picture of an ad campaign's performance if it is disproportionately influenced by heavy clickers unrepresentative of the target audience.