Knowing how people will use something is essential

User-centered design means working with your users all throughout the project.

The designer shouldn't think of a simple dichotomy between errors and correct behavior; rather, the entire interaction should be treated as a cooperative endeavor between person and machine, one in which misconceptions can arise on either side.

What makes something simple or complex? It's not the number of dials or controls or how many features it has: It is whether the person using the device has a good conceptual model of how it operates.

To me, error analysis is the sweet spot for improvement.

Learning should take place when it is needed, when the learner is interested, not according to some arbitrary, fixed schedule

The hardest part of design ... is keeping features out.