errors

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# Errors

Most "accidents" attributed to human error are actually design errors. The user took the action the design invited; the action's consequence wasn't what the user intended. Reframing errors as a *design* property — rather than a *user* failing — opens the design space dramatically: errors can be prevented, made impossible, made obvious, or made recoverable. The right strategy depends on the kind of error.

## Definition (in our own words)

An error is any action (or omission) that produces an unintended result. Errors come in two distinct kinds, and they need different design responses. **Slips** are skill-based failures: the user knew what to do but their action didn't match their intent — a typo, a wrong-button click, a slip of attention. **Mistakes** are knowledge-based failures: the user did exactly what they intended, but the intent was wrong because their model of the situation was wrong — they misread an alarm, misunderstood a diagram, made a wrong choice based on incomplete information. The two kinds happen for different reasons; they're prevented by different design moves.

## Origins and research lineage

- **Donald Norman**, "Categorization of Action Slips" (*Psychological Review*, 1981, vol. 88) and *The Design of Everyday Things* (1988). Norman introduced the slip/mistake distinction to design vocabulary and argued that error is a property of design, not user.
- **James Reason**, *Human Error* (Cambridge University Press, 1990). The standard reference work in human-error research, originally from industrial-safety domains (aviation, nuclear, medical). Reason expanded Norman's framework with detailed taxonomies (skill-based / rule-based / knowledge-based errors) and the "Swiss cheese" model of cascading failures.
- **Lidwell, Holden & Butler** (2003) compactly distinguished slips (errors of execution, automatic/unconscious) from mistakes (errors of intention, conscious mental processes). The book identifies two slip subtypes (Action, Attention) and three mistake subtypes (Perception, Decision, Knowledge).
- **Charles Perrow**, *Normal Accidents* (1984). Argued that complex tightly-coupled systems produce errors as an emergent property of system design, not as failures of individual operators. Influential in industrial-safety and software-system design.