Traditionally, problem difficulty has been characterized as how broad, or "branchy," a problem’s problem space is. The more branches in the problem space, the harder the problem.

So why is the Chinese Ring puzzle so hard?

Kotovsky & Simon (1990) showed that various isomorphs of the Chinese ring puzzle can be made more or less difficult than the basic problem.

The Chinese ring puzzle has a linear problem space, so clearly the problem space itself is not the source of the difficulty. What is?

Obviously, experts are better than novices at solving problems in their area of expertise. How does expertise affect problem solving?

Is it simply a matter of being able to execute procedures faster, having more relevant information available, or is there something about how experts represent the problem that aids their problem solving?

Physics problems — Chi et al.

Physics experts classify physics problems based on the underlying structure, e.g. conservation of energy problems, momentum problems, etc.

Novices classify based on surface structures, e.g. pulley problems, inclined plane problems, etc.

Remembering chess boards — Chase & Simon

Chess experts were better at remembering positions of pieces on the board when those positions made sense in the context of a larger game, but no better when the pieces were randomly placed.