Human Error
Error versus violation
An error is an unintentional deviation from intention or from the required standard. A violation is a deliberate deviation from a rule or procedure, made knowingly. Errors divide into: slips (the intention was right but the action was wrong, for example wrong switch or wrong torque), lapses (a planned action is forgotten), and mistakes (the plan itself was wrong - rule-based when a known rule is misapplied, knowledge-based when reasoning fails in a novel situation).
Violations divide into routine (the accepted way of working), situational (pressure, impossible procedures or time) and exceptional (rare, extreme circumstances).
Latent conditions and the error chain
Reason's model: latent conditions from management, design and procurement combine with active failures at the sharp end. Accidents are usually chains of small events, so breaking any single link - a cross-check, a second signature, a walk-round - prevents the outcome. The 'dirty dozen' list (fatigue, pressure, distraction, lack of resources, and so on) is a practical memory aid for precursors.
Error management
Reduce error through design and training; contain it through cross-checks, inspection and independent certification; tolerate it through redundancy. Reporting systems and a just culture supply the learning.
Traps
Assuming all violations deserve punishment, or that an error proves incompetence. Routine violations often indicate a procedure that does not match the real task.
Rule of thumb: right action, wrong execution - slip; correct execution of a wrong plan - mistake; deliberately not following the rule - violation.