Distinguish slips from mistakes and apply different prevention

What it is

A slip preserves the right intention but executes it wrongly: tapping an adjacent button, dragging to the wrong list, or entering data in a duplicate field. A mistake means the intention or plan itself fits the situation poorly: misunderstanding a rule, choosing the wrong method, or trusting a bad default. They have different causes, and “confirm again” is not a universal cure.

Why it happens

Slips arise in skilled action and similar interfaces, so prevention uses physical separation, spatial and sequential mapping, distinguishable targets, read-back, and immediate review. Mistakes arise in understanding and judgment, so users need domain explanation, comparison of consequences, dry runs, reversible scope, and expert review; another confirmation cannot repair a wrong mental model and becomes noise.

Studying it

Use think-aloud, logs, and replay to classify events as correct intention with execution deviation, faulty intention, system fault, or external constraint. Record task stage, experience, interface similarity, time pressure, and recovery. For each design, ask counterfactually: if a slip, would changing action targets eliminate it? If a mistake, would explaining the rule change the choice?

Where it stops holding

Classification changes with evidence; one event can include both execution deviation and planning error. Stress, fatigue, language, and expertise alter their proportions. Do not classify every user action as user error; ambiguous systems and upstream data also produce slips and mistakes.

Applying it

• Maintain an event taxonomy counting slips, mistakes, system faults, and external constraints separately.
• For slips, add target spacing, grouping, read-back, separation of hazardous zones, and undo; for mistakes, add rule explanation, comparative preview, and dry runs.
• Avoid one generic dialog for every error; messages should state which kind of problem the system detected.
• After fixes, track category migration to ensure slips have not become mistakes or created new workarounds.

Related

• Same group: B3.05.1 Eliminating error conditions beats providing error messages · B3.05.3 High-consequence operations need extra confirmation or delay
• Nearby: A10.01 Distinguishing slips, mistakes, and violations · Y3 Error Prevention and Recovery
• Search terms: slips and mistakes · human error · error taxonomy