Human Factors and Ergonomics
Human factors applies human capabilities and limitations to work-system design. Candidates must know anthropometry (design for the 5th–95th percentile range; reach limits use the 5th percentile, clearance uses the 95th), biomechanics (NIOSH lifting equation, recommended weight limit, lifting index ≤ 1.0), and work physiology (energy expenditure, rest allowances).
The NIOSH lifting equation computes RWL = LC × HM × VM × DM × AM × FM × CM, where LC = 51 lb (23 kg) and multipliers adjust for horizontal distance, vertical location, distance, asymmetry, frequency, and coupling. The lifting index = load/RWL; values above 1.0 indicate increased risk. Traps: forgetting the frequency multiplier or coupling multiplier, and using the wrong constant.
Cognitive and display/control design: Fitts' law relates movement time to distance and target width; signal detection theory separates sensitivity (d') from response bias; vigilance degrades over time. Controls should be compatible with displays (population stereotype), and emergency controls should be distinct. Know the design of displays (visual vs. auditory), coding (color, shape), and the limits of absolute judgment (about 7 ± 2 categories).
Environmental factors: noise (dose and permissible exposure limits), illumination (footcandles by task), thermal comfort (WBGT for heat stress), and vibration. Traps: confusing percentile directions; assuming average-sized design fits everyone (it fits almost no one); and misapplying the lifting equation to one-handed or unstable loads without noting its assumptions.
Worked example
Load = 40 lb; multipliers HM = 0.83, VM = 0.99, DM = 0.93, AM = 1.0, FM = 0.88, CM = 1.0. RWL = 51 × 0.83 × 0.99 × 0.93 × 1.0 × 0.88 × 1.0 ≈ 34.3 lb. LI = 40/34.3 ≈ 1.17 → risk exceeds the guideline; redesign the lift.