  We present an empirical study in which model-based testing (MBT) was applied
to a mobile system: the Android client of QuizUp, the largest mobile trivia
game in the world. The study shows that traditional MBT approaches based on
extended finite-state machines can be used to test a mobile app in an effective
and efficient way. Non-trivial defects were detected on a deployed system that
has millions of users and was already well tested. The duration of the overall
testing effort was of three months, including the construction of the models.
Maintaining a single behavioral model for the app was key in order to test it
in an efficient way.
