It's tough to measure mobile app success, but Forrester has come out with a very good framework for doing just that: The App Engagement Index. It's a great tool for gauging the success of media, communications, and entertainment apps, although it's not as useful for purely business apps such as airline and banking ones. The index is based on a simple premise: The more engaged a customer is with an app, the more effective the app is. To create the index, Forrester engagement and behavioral for more than 100 apps. Based on what it found, Forrester created four separate engagement indicators: popularity, commitment, frequency, and time spent. Popularity is a simple metric, and measures how popular the app is among smartphone users. Commitment measures, in Forrester's words, "the share of users that access the app on a weekly basis. A high share indicates that the app is able to maintain a steady portion of returning users. Frequency measures how many days an average user accesses the app over a specific period of time. The more often people use the app, the more value they get from it. Time spent measures the average amount of time that a users spends with the app. Again, the more time a user spends with an app, the more successful the app is in engaging him.