Work study and ergonomics

In short

Work study is the systematic examination of how work is done, in order to improve the method (method study) and to establish how long the job should take (work measurement). Ergonomics (human factors) fits the job, the machine and the workplace to the worker's body and mind, rather than forcing the worker to adapt. Together they fix the industrial engineer's two core outputs: a standard method and a standard time, which then feed line balancing, capacity planning, incentive wages, costing and labour budgets. The topic is examined in GATE (ME and PI), every university Industrial Engineering paper, and asked in interviews at Maruti Suzuki, Tata Motors, Bajaj, Hero, TVS, Ashok Leyland, L&T, Titan, Amazon, Flipkart and Delhivery for IE/plant/operations roles.

Explained

Productivity, work study and work content

• Productivity = output / input. Partial productivity uses one input (labour, machine, material, energy); total factor productivity uses all. Rising productivity means more output per unit input, not more effort.
• Total time for a job = basic work content (the irreducible minimum) + excess work content due to poor design/material + excess due to poor methods + ineffective time due to management + ineffective time due to the worker.
• Method study attacks the excess work content; work measurement exposes the ineffective time. This is the standard ILO breakdown (ILO, *Introduction to Work Study*) that most Indian syllabi follow.
• Work study = method study + work measurement. Benefits: higher productivity at low capital cost, better layouts, fair wage incentives, reliable delivery dates. Limitations: resistance from workers (seen as speed-up), needs trade-union consultation, poor at highly variable or creative work, and standards drift as technology changes.

Method study - the eight steps and recording tools

1. SELECT the job - high labour content, bottleneck, costly, unsafe, or high scrap.
2. RECORD the present method using charts and diagrams (below).
3. EXAMINE critically with the questioning technique: purpose (what, why), place (where), sequence (when), person (who), means (how) - each asked as *what is done / why / what else could be done / what should be done*.
4. DEVELOP the improved method (ELIMINATE, COMBINE, REARRANGE, SIMPLIFY - the ECRS rule; eliminate first, it pays most).
5. EVALUATE alternatives on cost, safety, quality, feasibility.
6. DEFINE the new method in a written standard practice sheet.
7. INSTALL - train operators, get supervisor and union acceptance.
8. MAINTAIN - audit periodically so the method does not slip back.

(Several textbooks compress this to six steps: SREDIM - Select, Record, Examine, Develop, Install, Maintain.)

• Five ASME process-chart symbols: circle = operation, arrow = transport, square = inspection, D = delay, inverted triangle = storage. A combined activity is shown by overlapping symbols.
• Charts for a sequence: *outline process chart* (operations and inspections only), *flow process chart* (man-, material- or equipment-type, all five symbols, with distances and times), *two-handed (left-hand right-hand) chart* for short-cycle bench work.
• Charts against a time scale: *multiple activity chart* and *man-machine chart* (shows idle time of operator vs machine; used to decide how many machines one operator can tend), *SIMO chart* (simultaneous motion cycle chart - therbligs of both hands against time, from a film/video).
• Diagrams showing movement: *flow diagram* (path on a scaled layout), *string diagram* (thread on a scale model measures distance travelled), *travel chart / from-to chart* (matrix of movements between stations), *cyclegraph and chronocyclegraph* (light on the hand photographed - the chronocyclegraph's pear-shaped dots show direction and speed).

Motion study, therbligs and motion economy

• Motion study (Frank and Lillian Gilbreth) analyses the elemental hand and body motions. Micromotion study uses film at a known frame rate; memomotion study uses slow filming (1 frame per second or slower) for long, irregular work.
• Therbligs: 17-18 basic elements (the count differs by text), each with a symbol and colour - search, find, select, grasp, hold, transport loaded, transport empty, position, pre-position, assemble, disassemble, use, release load, inspect, plan, unavoidable delay, avoidable delay, rest to overcome fatigue.
• Effective therbligs (reach, grasp, move, use, release) do the work; ineffective therbligs (search, select, position, hold, delays) should be eliminated or reduced - pre-positioning jigs and gravity feed chutes remove them.
• Principles of motion economy - three groups:
  • *Use of the human body:* both hands should start and finish together and not be idle at the same time; motions should be symmetrical and simultaneous; momentum should be used; continuous curved motions beat zig-zag straight lines; ballistic motions beat controlled ones; the lowest classification of movement should be used (class 1 finger, 2 wrist, 3 forearm, 4 upper arm, 5 whole trunk).
  • *Workplace arrangement:* fixed places for all tools and materials; tools and materials inside the normal working area (forearm sweep, about 350-400 mm radius) and within the maximum working area (full arm, about 550-650 mm); gravity bins and drop-delivery chutes; good lighting; correct seat and bench height, with sit-stand options.
  • *Design of tools and equipment:* combine two tools into one; relieve the hands using jigs, fixtures and foot-operated devices; shape handles for maximum contact; pre-position tools; set lever and wheel positions for least body movement.

Work measurement - techniques and time study

• Purpose: set standard times for planning, scheduling, line balancing, manpower estimation, costing, incentive schemes and comparing methods.
• Techniques: (a) time study with a stopwatch, (b) work sampling (ratio-delay), (c) predetermined motion time systems (PMTS) - MTM-1/2/3, Work-Factor, MOST, MODAPTS, (d) synthesis / standard data from element banks, (e) analytical estimating for non-repetitive maintenance-type work.
• Time study procedure: select the job → select a qualified, trained, average-paced operator → break the job into elements (repetitive, occasional, constant, variable, machine, governing, foreign) with clear break-points → decide number of cycles → time with a stopwatch (fly-back/snap-back reading, or continuous reading and subtract, or differential/cumulative timing) → rate the operator's performance → compute normal time → add allowances → standard time.
• Performance rating brings an observed time to the pace of a qualified worker working at 100%. Scales: British/BSI 0-100 (100 = standard), 60-80 (80 = standard, older Bedaux), 75-100 (100 = standard). Methods: speed rating, Westinghouse four-factor (skill, effort, conditions, consistency), synthetic rating (from PMTS elements), objective rating (speed plus job difficulty).
• Allowances:
  • *Relaxation (rest) allowance* = personal needs (about 5%, often 7% for women in Indian practice) + basic fatigue (about 4%) + variable fatigue (posture, force, heat, humidity, noise, light, mental strain - from allowance tables).
  • *Contingency / unavoidable delay allowance* - usually up to 5% for small irregular work such as tool sharpening.
  • *Process / unoccupied-time allowance*, *special allowances* (set-up, tool change, cleaning, learning), *policy (incentive) allowance*.
• Work sampling: a large number of instantaneous random observations estimates the proportion of time spent on each activity. Cheaper than time study for long-cycle, group or indirect work (maintenance, nursing, clerical, materials handling), needs no stopwatch, is less resented - but gives no element detail, no method record, and is uneconomical for short-cycle repetitive jobs. Observations must be randomly timed, not at a fixed interval, or the estimate is biased.
• PMTS: elemental times from published tables; MTM uses TMU (time measurement unit) where 1 TMU = 0.00001 h = 0.036 s, so 1 s = 27.8 TMU. Advantages - no rating needed, method can be timed before the job exists, consistent standards. Limitations - costly licensed training, suits short-cycle manual work, not machine-paced or heavy work.

Ergonomics - the man-machine system

• Ergonomics (Greek *ergon* work + *nomos* law), or human factors, designs tasks, tools, workstations and environments around human capabilities and limits. Divided into physical (posture, loads, repetition, layout), cognitive (displays, controls, mental workload, error) and organisational (shifts, teams, work-rest cycles) ergonomics.
• Man-machine system: the human senses a display, decides, and acts on a control; the machine responds and feeds back. Assign to the machine what is fast, repetitive, forceful and precise; to the human what needs judgement, pattern recognition and improvisation.
• Anthropometry - body measurements used for design. Design rules: *design for extreme* (clearance - use the 95th percentile male, e.g. doorway and legroom; reach - use the 5th percentile female, e.g. control location), *design for adjustability* (chair and seat height, 5th-95th percentile), *design for the average* only where neither applies (e.g. a counter height). IS 7250 covers basic human body measurements for technological design; Indian population dimensions are smaller than the Western data in many handbooks, so imported machine guards and reach distances often fail here.
• Posture and manual handling: static muscle loading cuts blood flow and tires quickly; sitting suits precision and light work, standing suits force and movement, sit-stand suits long shifts. Keep work at or slightly below elbow height (lower for heavy work, higher for precision), avoid twisting, over-reaching and overhead work, and keep the back's natural curve.
• Work physiology: energy expenditure in kcal/min or kJ/min; heart rate and oxygen consumption measure workload. A commonly used sustainable limit for an 8-hour shift is about 4-5 kcal/min for men (lower for women); above that, rest allowance must rise. Maximum aerobic capacity declines with age; recovery heart rate is a practical field measure.
• MSDs: musculoskeletal disorders - low back pain, carpal tunnel syndrome, tenosynovitis, tennis elbow, shoulder tendinitis - caused by force, awkward posture, repetition, vibration and cold, acting over time.
• Assessment tools: RULA (rapid upper limb assessment, score 1-7, action levels 1-4), REBA (whole body, includes legs and load), OWAS (posture sampling, four action categories), QEC, Strain Index, NIOSH lifting equation (gives a recommended weight limit and lifting index), Snook tables for push/pull/carry.
• Environment: illumination (IS 3646 / National Building Code recommends roughly 100-200 lux for circulation, 300-500 lux for general assembly work and 1,000 lux or more with local lighting for fine inspection - check the current edition), noise (Factories Act / Model Rules limit 90 dB(A) for 8 h, with the exchange rate halving exposure time per 3-5 dB rise depending on the rule applied; 115 dB(A) is the ceiling for short exposure), thermal comfort (WBGT index, air movement, humidity), vibration (hand-arm and whole-body), colour coding and safety signs (IS 9457, IS 12349).
• Indian legal frame: the Factories Act 1948 - still the examined reference - fixes 48 hours a week, 9 hours a day, a half-hour rest after 5 hours of continuous work, a weekly holiday, and limits on maximum permissible loads for men, women and adolescents under state Factories Rules (typically of the order of 50-55 kg for adult men, with lower limits for women and young persons - the exact figures are set by each state's rules, so quote the applicable state rule). The Occupational Safety, Health and Working Conditions Code, 2020, which consolidates the Factories Act and 12 other central labour laws, was brought into force with the other three labour codes on 21 November 2025, repealing the Factories Act; central and state rules under it are still being notified, so check the applicable rules (and the 8-hour normal working day it sets) before quoting figures for compliance. BIS issues the IS standards; DGFASLI and state Factories Inspectorates enforce; ISO 6385 gives ergonomic principles for work-system design.

Comparing and applying the outputs

• A standard time is valid only for the specified method, the specified material and a qualified worker; change any one and the standard must be re-set.
• Standard time feeds: capacity (machines/operators needed), line balancing (cycle time, number of stations, balance efficiency), incentive wages (Halsey, Rowan, Taylor piece rate), standard costing and labour variance, delivery promises and make-or-buy labour costing.
• Learning curve: repeated work gets faster; standards should be set after learning has flattened, or a learning allowance applied.

Key points

• Work study - method study (how the work is done) plus work measurement (how long it should take).
• Method study aim - eliminate unnecessary work; work measurement aim - set a standard time and reveal ineffective time.
• SREDIM / 8 steps - Select, Record, Examine, Develop, Evaluate, Define, Install, Maintain.
• ECRS - Eliminate, Combine, Rearrange, Simplify; elimination gives the largest saving at least cost.
• Five chart symbols - operation (circle), transport (arrow), inspection (square), delay (D), storage (inverted triangle).
• String diagram - measures distance travelled; travel chart - frequency of movement between stations; flow diagram - path on a scaled layout.
• Man-machine chart - time-scaled chart of operator vs machine idle time; fixes how many machines one operator can run.
• Therbligs - 17-18 basic motions; ineffective ones (search, select, hold, position, delays) are the targets.
• Motion economy - both hands busy and symmetrical, lowest movement class, fixed tool locations, gravity feed, jigs instead of holding.
• Normal time = observed time × rating; standard time = normal time + allowances.
• Rating - judging pace against 100% (BSI scale); Westinghouse rates skill, effort, conditions, consistency.
• Relaxation allowance = personal (~5%) + basic fatigue (~4%) + variable fatigue from conditions.
• Work sampling - random instantaneous observations; best for long-cycle, group and indirect work; needs no rating of pace to get activity proportions.
• PMTS (MTM) - table-based times, 1 TMU = 0.036 s; no rating needed and can be applied before the job exists.
• Anthropometry rules - design for extreme (clearance 95th percentile, reach 5th percentile), for adjustability, for average only as a last resort; IS 7250 gives Indian body data.
• Ergonomic risk tools - RULA (upper limb, 1-7), REBA (whole body), OWAS (posture sampling), NIOSH equation (RWL and lifting index).
• Indian law - Factories Act 1948 (48 h/week, 9 h/day, rest after 5 h, 90 dB(A) noise limit, state load limits) is the examined reference; the OSH Code 2020 that consolidates it came into force on 21 November 2025, with rules still being notified.
• Standard time is method-specific - a new method or material invalidates the old standard.

Formulas and examples

Example 1 - Normal time, standard time and output. A stopwatch study of a drilling job gives cycle times 0.52, 0.55, 0.50, 0.58, 0.54, 0.51 min. The operator is rated at 110% (BSI scale). Allowances: personal 5%, basic fatigue 4%, variable fatigue 6%, contingency 3%.

• OT = (0.52 + 0.55 + 0.50 + 0.58 + 0.54 + 0.51)/6 = 3.20/6 = 0.5333 min
• NT = 0.5333 × 1.10 = 0.5867 min
• Total allowance = 5 + 4 + 6 + 3 = 18%
• ST = 0.5867 × 1.18 = 0.6923 min ≈ 0.692 min/piece
• Standard output in a 480-min shift = 480 / 0.6923 = 693 pieces (use 450 effective min if the shift has a 30-min unpaid break: 450/0.6923 = 650 pieces).

Example 2 - Number of cycles to observe. From a pilot study of 10 cycles, mean x̄ = 1.80 min, standard deviation s = 0.21 min. Required: 95% confidence, ±5% relative accuracy.

• n = (1.96 × 0.21 / (0.05 × 1.80))² = (0.4116 / 0.09)² = (4.573)² = 20.9 → 21 cycles
• At ±10% accuracy: n = (4.573/2)² = (2.287)² = 5.2 → 6 cycles. Tighter accuracy costs roughly four times the observations for half the error.

Example 3 - Work sampling. A 40-hour work-sampling study of a packing section (5 workers) takes 1,200 random observations; 960 show productive work, 240 show idle/delay. Output in the period = 7,200 cartons. Average rating during productive observations = 95%. Allowances = 15%.

• Proportion working p = 960/1,200 = 0.80 (80%); idle = 20%.
• Total man-minutes = 5 workers × 40 h × 60 = 12,000 man-min; productive man-min = 0.80 × 12,000 = 9,600.
• Normal time per carton = (9,600 × 0.95)/7,200 = 9,120/7,200 = 1.267 man-min
• Standard time = 1.267 × 1.15 = 1.457 man-min per carton
• Sample size check at 95% confidence, ±5% relative error: n = (1.96)²(1 − 0.80)/(0.80 × 0.05²) = 3.8416 × 0.20/(0.80 × 0.0025) = 0.76832/0.002 = 384 observations - so 1,200 observations comfortably meets the requirement (for ±2% absolute error: n = 3.8416 × 0.8 × 0.2/0.0004 = 1,537, which 1,200 does not meet).

Example 4 - Man-machine chart and machine assignment. An operator loads and unloads a CNC machine in 1.5 min (l), the machine then runs unattended for 6.0 min (m), and walking between machines takes 0.5 min (w).

• Ideal machines per operator n = (l + m)/(l + w) = (1.5 + 6.0)/(1.5 + 0.5) = 7.5/2.0 = 3.75 → assign 3 or 4.
• With n = 3: cycle = l + m = 7.5 min; operator works 3 × (1.5 + 0.5) = 6.0 min, idle 1.5 min → output 3 pieces/7.5 min = 0.40 pieces/min; operator utilisation 6.0/7.5 = 80%, machine utilisation 100%.
• With n = 4: operator is the bottleneck, cycle = 4 × (1.5 + 0.5) = 8.0 min; output 4 pieces/8.0 min = 0.50 pieces/min; operator utilisation 100%, machine utilisation 7.5/8.0 = 93.75% (each machine idle 0.5 min).
• Choose on cost: n = 4 gives 25% more output but idles costly machines; compare operator wage against machine hourly rate.

Example 5 - NIOSH lifting equation. A worker lifts 12 kg boxes from a pallet. Multipliers from the study: HM = 0.80, VM = 0.93, DM = 0.88, AM = 0.90 (30° twist), FM = 0.75 (1 lift/min over a full shift), CM = 0.95 (fair coupling, low lift).

• RWL = 23 × 0.80 × 0.93 × 0.88 × 0.90 × 0.75 × 0.95 = 23 × 0.4198 = 9.66 kg
• LI = 12 / 9.66 = 1.24 → above 1, so redesign is needed.
• Fixes: bring the pallet closer and raise it on a scissor lift (HM → 1.0, VM → 1.0) and remove the twist (AM → 1.0): RWL = 23 × 1.0 × 1.0 × 0.88 × 1.0 × 0.75 × 0.95 = 14.42 kg, LI = 12/14.42 = 0.83 - acceptable.

Where it is used

• Automobile and auto-component assembly: Maruti Suzuki, Tata Motors, Hero, Bajaj, Bosch and Sundram Fasteners use time standards for line balancing, takt time, man-power per shift and ergonomic scoring (RULA/REBA) of assembly stations.
• Warehousing and e-commerce fulfilment: Amazon, Flipkart, Delhivery and BigBasket set pick-rate standards by work sampling and time study, and design pick faces, trolley heights and lifting rules to limit back injury.
• Textiles, garments and footwear: Arvind, Shahi Exports and Bata use SAM (standard allowed minutes) per operation, GSD/PMTS data, operator ratings and sewing-station ergonomics for efficiency and incentive pay.
• Steel, cement and heavy fabrication: Tata Steel, JSW and L&T use analytical estimating and work sampling for maintenance crews, plus WBGT heat stress and noise limits under the factory safety rules.
• Healthcare, banking and BPO: nursing activity sampling in hospitals, teller and back-office transaction standards, and VDU workstation ergonomics (chair, monitor height, keyboard, lighting).
• Job roles: industrial engineer, IE executive (apparel), process/manufacturing engineer, work-study officer in PSUs and railways, EHS officer, consultant in operations improvement.

Interview questions

• Q: What is the difference between method study and work measurement? A: Method study finds the best way to do a job by eliminating unnecessary work; work measurement establishes how long that best method should take for a qualified worker.
• Q: Why must method study be done before work measurement? A: A standard time is valid only for a defined method, so timing a wasteful method just standardises the waste.
• Q: Define normal time and standard time. A: Normal time is the observed time adjusted for the operator's pace (observed time × rating); standard time adds relaxation, contingency and other allowances to normal time.
• Q: What is performance rating and why is it subjective? A: It is the analyst's judgement of the operator's pace against a defined 100% standard pace; since it relies on comparison with a mental benchmark, two analysts can differ, which is why rating films and periodic rating practice are used.
• Q: When would you choose work sampling over a stopwatch study? A: For long or irregular cycles, indirect and group work such as maintenance or materials handling, where you want activity proportions and machine utilisation cheaply and without continuous observation.
• Q: How many observations does a work sampling study need? A: From n = Z²(1−p)/(p·r²) for relative error, or Z²p(1−p)/e² for absolute error - for example about 384 observations for p = 0.8 at 95% confidence and ±5% relative error.
• Q: What is a man-machine chart used for? A: To plot operator and machine activity against time, expose idle time on either side, and decide how many machines one operator can be assigned.
• Q: State three principles of motion economy. A: Both hands should begin and end motions together and never be idle at the same time; motions should be symmetrical, curved and continuous rather than zig-zag; tools and materials should have fixed locations within the normal working area.
• Q: What are therbligs, and which ones do you try to eliminate? A: The basic hand motions named by the Gilbreths (17-18 of them); the ineffective ones - search, select, position, hold, plan and delays - are eliminated using jigs, pre-positioning and gravity feed.
• Q: What is ergonomics, and give one example of a change it would suggest? A: Fitting the job to the worker's physical and mental capabilities - for example raising a pallet on a lift table so parts are picked near elbow height instead of bending to floor level.
• Q: What does a RULA score or a NIOSH lifting index above 1 tell you? A: A high RULA score (5-7) means the posture needs investigation or immediate change; LI > 1 means the load exceeds the recommended weight limit and the lift must be redesigned.
• Q: Which Indian law governs working hours, rest and noise exposure in a factory? A: The Factories Act 1948 with state Factories Rules - 48 hours a week, 9 hours a day, rest after 5 hours of continuous work, and a 90 dB(A) limit for 8-hour noise exposure; these were consolidated into the OSH Code 2020, in force since 21 November 2025, under which the detailed rules are still being notified.
• Q: An operator is rated 80% - will the standard time be higher or lower than the observed time? A: Normal time is lower - 0.80 × observed time, since the operator is working slower than standard pace; adding allowances to that normal time still leaves the standard time below the observed time unless total allowances exceed 25%.

Common mistakes

• Multiplying or dividing by rating in the wrong direction - rating above 100% raises normal time above observed time, below 100% lowers it.
• Mixing the two allowance conventions: ST = NT × (1 + a) when the allowance is a fraction of normal time, but ST = NT/(1 − a) when it is a fraction of total time; state which convention is used.
• Taking observations in a work-sampling study at fixed intervals - the times must be random, or the result is biased by the work's own rhythm.
• Timing an unqualified, learner or deliberately slow operator and calling the result a standard, or using one "fast" worker's time as the standard.
• Forgetting that the standard time belongs to the recorded method - quoting an old standard after the method, tool or material changed.
• Designing a workstation for the "average" person, or using Western anthropometric tables for Indian workers, instead of using 5th/95th percentile and adjustability with IS 7250 data.
• Confusing the chart symbols - treating inspection as an operation, or a controlled storage as a delay - and leaving out distances and times on a flow process chart.

Related topics

Production planning and control · Lean manufacturing and Six Sigma · Plant layout and material handling · Industrial safety and maintenance · Operations research basics · Engineering economics and costing