Error Taxonomy: Slips vs Mistakes
Norman & James Reason's classification. Error = any deviance from appropriate behavior. Two major classes:
Slips (execution is flawed; goal is correct)
You intend one action and do another. More common in experts (skilled actions run subconsciously, so attention lapses). Two classes:
- Action-based slip — wrong action performed (poured milk in coffee, then put the cup in the fridge).
- Memory-lapse slip — intended action not done / result not checked (forgot to turn off the burner).
Three action-slip types most relevant to design:
- Capture slip — a more frequent/recent action captures the intended one when they share an opening sequence (counting "…10, Jack, Queen, King"). → design sequences to differ from the very start.
- Description-similarity slip — correct action on the wrong (similar) object (sweaty shirt into the toilet, not the laundry). → make controls/displays distinct (cockpit shape-coding).
- Mode error — same control means different things in different modes; user thinks the system is in another mode. Inevitable when actions outnumber controls (Airbus vertical-speed vs descent-angle: −3.3 → fatal). → avoid modes, or make the current mode obvious.
Mistakes (the plan/goal itself is wrong)
A poor decision/diagnosis; actions may execute perfectly but serve the wrong plan. More common in novices. Result from conscious deliberation, over-generalization. Three classes:
- Rule-based — situation diagnosed correctly, but the wrong rule chosen.
- Knowledge-based — situation misdiagnosed due to wrong/incomplete knowledge (Gimli Glider: fuel computed in pounds, not kilograms).
- Memory-lapse mistake — forgetting at the goal/plan/evaluation stage.
Mapping to the action cycle
Slips arise in the lower stages (perform/perceive/interpret); mistakes in the top three (goal/plan/compare). Memory lapses occur at the transitions between stages — high transitions → mistakes, low → slips. See The Seven Stages of Action.
Note: deliberate violations are intentional, known-risky deviations — organizational/societal, not classed as errors here.
Confidence
1.0 — explicit, widely cited taxonomy.
Sources: ["DOET ch.5 'Two Types of Errors' p.170-186 (Norman & Reason)"]