Richard Bartle presents a model of "non-player types" to explain why people may stop playing or not play a game in the first place. The model proposes two axes - challenge on the x-axis, and meaning/depth on the y-axis. Games that are too easy, hard, shallow, or deep can cause players to lose engagement. Specific quadrants of the graph are used to illustrate examples of games that may fail to retain players due to the right balance not being struck between challenge and meaning. The model is then applied to suggest some players may be willing to make microtransactions in free-to-play games to alter the challenge or depth to better suit their preferences.