This talk will focus on how to use telemetry, statistical analysis, A/B testing and multivariate testing to optimize player retention in online and social games. We will show how we used these methods to attract and retain 1.5 million players in three online games: Refraction, Hello Worlds, and Foldit. We will present data from 110,000 players showing that music and sound effects had no effect on player retention, that secondary objectives caused as many as 40% of players to quit prematurely, and that text tutorials were only effective in the most complex and least conventional game of the three.