Tasks
Categories of task
Physical work involves lifting, manual handling and ergonomics - poor posture and excessive load cause injury and reduce precision. Repetitive and vigilance tasks are deceptive: inspection is never perfectly reliable, and even trained inspectors miss defects, especially rare ones, with performance declining over long sessions. Complex tasks place heavy demands on memory, planning and decision making, and are the most vulnerable to interruption.
Skill, rule and knowledge
Rasmussen's classification is worth knowing because it predicts error type. Skill-based performance is automatic and produces slips and lapses. Rule-based performance follows if-then procedures and produces rule-based mistakes when the wrong rule is chosen or the rule is misapplied. Knowledge-based performance handles novel situations and produces knowledge-based mistakes when reasoning is faulty or information is missing.
Task analysis and design
Task analysis breaks a job into steps, identifies hazards and dependencies, and informs procedures, tooling, staffing and training. Good task design gives clear instructions, the correct tools, enough time, defined authority and freedom from unnecessary interruption.
Traps
The frequent false statement that thorough inspection finds all defects, or that boredom is harmless because nothing is happening. Also confusing a lapse (forgotten action) with a slip (wrong action).
Rule of thumb: if a person knows the right procedure but executes it wrongly, think slip; if the plan itself was wrong, think mistake.