Gamedev website Gamasutra has a feature up that puts into words a concept I've been struggling to explain for years. The author modestly calls it "Ken Perlin's Law", but I venture that his own term, "credibility budget", is far more catchy:

Ken Perlin's Law: The cost of an event in an interactive story should be directly proportional to its improbability.

[...] What is the unit of cost of an improbable event in a story? Its credibility. That's what gets spent when something improbable happens. And in fact, every story, interactive or non-interactive, book, movie, television, or computer game, has a credibility budget. The story itself can only tolerate a certain amount of improbability before the credibility budget is exhausted, and the story is ruined.

The author goes on to point out that unlike in books and movies, where credibility is the sole domain of the creator, video games put the responsibility into both creator and player's hands. If either one of us overdraws the credibility budget, the narration collapses, and the story is "broken".

Grand Theft Auto is used as an example of a game with a well-managed credibility budget. The world is open, but solid. The player is not allowed to break the story, but rarely feels as if he is being railroaded, either.
So this is my official notice. The term "credibility budget" is to be entered into geek phraseology immediately. I will brook no arguments. Eliza Gauger

The Designer's Notebook: Introducing Ken Perlin's Law [Gamasutra]

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