The ELO ranking is more suited to explicit head-to-head matchups, but its statistical basis could be adapted to app rankings. You could basically collect statis

The ELO ranking is more suited to explicit head-to-head matchups, but its statistical basis could be adapted to app rankings. You could basically collect statistics for each app position (#1 averages 1000 purchases/day, #2 averages 500, etc.) and then estimate the quality of each app based on how its purchase rate differs from the expected average for that position. Google and other advertisers face a similar problem with sorting ads, which suggests some other relevant inputs: http://en.wikipedia.org/wiki/Quality_Score http://en.wikipedia.org/wiki/Quality_Score