Statistical Modelling of Level Difficulty in Puzzle Games
  Successful and accurate modelling of level difficulty is a fundamental
component of the operationalisation of player experience as difficulty is one
of the most important and commonly used signals for content design and
adaptation. In games that feature intermediate milestones, such as completable
areas or levels, difficulty is often defined by the probability of completion
or completion rate; however, this operationalisation is limited in that it does
not describe the behaviour of the player within the area.
  In this research work, we formalise a model of level difficulty for puzzle
games that goes beyond the classical probability of success. We accomplish this
by describing the distribution of actions performed within a game level using a
parametric statistical model thus creating a richer descriptor of difficulty.
The model is fitted and evaluated on a dataset collected from the game Lily's
Garden by Tactile Games, and the results of the evaluation show that the it is
able to describe and explain difficulty in a vast majority of the levels.
