Mapping the many faces of "retention"

Everybody knows what "retention" means until they have to design a metric. Ken Rudin, once of LucidEra and now general manager of analytics at the games site Zynga, thought that he and his team could "put something together" quickly — but it actually took "four solid weeks of discussion and debate."

About 50 million people play Zynga games every day. It'…