Work Design and Ergonomics
Methods analysis, work measurement, and fitting the task to the human.
Methods analysis
Use flow process charts and operation analysis, then eliminate, combine, rearrange, and simplify. Principles of motion economy: use both hands, work with natural rhythm, use gravity feed, place tools within the normal work area, and use foot controls only where appropriate.
Time study
Observed time × performance rating = normal time. Standard time = normal time × (1 + allowance fraction). Allowances cover personal, fatigue, and delay (PFD) needs, commonly 10–20%. The allowance is applied to normal time, never to the observed time. Work sampling estimates the proportion of time spent in an activity; sample size n = z²p(1−p)/e². Predetermined time systems (MTM, MOST) build standards from elemental motions without a stopwatch.
Learning curves
Y = aX^b where b = ln(learning curve %)/ln 2. On an 80% curve, doubling cumulative output reduces unit time to 80% of its prior value. Unit-time and cumulative-average formulations differ - check which one the question asks for.
Ergonomics
Anthropometry: design for the 5th percentile for reach and the 95th percentile for clearance - never the average. The NIOSH lifting equation gives RWL = 23 × HM × VM × DM × AM × FM × CM, with lifting index = load ÷ RWL; an index above 1 signals elevated risk. RULA and REBA score postural risk. Cumulative trauma disorders are driven by repetition, force, awkward posture, vibration, and insufficient recovery time.
Traps
Applying allowances to observed time; using the 50th percentile for a reach; forgetting that predetermined time systems already include allowances; mixing unit and cumulative-average learning formulas.
Worked example
Observed 1.20 min per cycle at 110% rating gives normal time = 1.32 min. With a 15% PFD allowance, standard time = 1.32 × 1.15 = 1.518 min, about 39.5 cycles per hour.