Control Measures - Time and Motion Study, Method Study, Work Study: formula sheet

Key formulas

Work study structure

Work study = Method study + Work measurement

Method study decides how the job is done. Work measurement decides how long it should take.

Basic procedure (SREDDIM)

Select → Record → Examine → Develop → Define → Install → Maintain

A common seven-step version. Some books combine steps. Learn the order and give a one-line purpose for each.

Productivity

Productivity = Output ÷ Input

Work study raises this ratio by cutting wasted input or raising output.

Method study procedure (order of steps)

Select → Record → Examine → Develop → Define → Install → Maintain

Learn the sequence as written. A common memory line: Some Real Experts Develop Defined Improved Methods.

Critical examination: primary questions

Purpose (What is done? Why?) · Place (Where? Why?) · Sequence (When? Why?) · Person (Who? Why?) · Means (How? Why?)

Primary questions establish the facts of the activity. Ask Why? of each answer to test whether it is really needed.

Critical examination: secondary questions

What else could be done? · Where else? · When else? · Who else? · How else?

Secondary questions explore alternatives so that you can select the best option. Follow them with the ECRS idea: Eliminate, Combine, Rearrange, Simplify.

Percentage saving from a new method

Saving % = (Old cost or time − New cost or time) ÷ Old cost or time × 100

Use to show the benefit of the improved method. Use the same unit for old and new.

Operation

Symbol: circle (○)

Value-adding step where material is changed or assembled. Also used for preparing or giving information.

Transport

Symbol: arrow (→)

Movement of worker, material or equipment from one place to another, except during an operation.

Inspection

Symbol: square (□)

Checking quality or quantity against a standard.

Delay (temporary storage)

Symbol: D

Unplanned wait where the next step cannot start immediately.

Storage (permanent)

Symbol: inverted triangle (▽)

Material is held and needs authorisation to be issued.

Combined activity

Symbol: circle inside a square (⊙ in □)

Operation and inspection done together by the same person at the same place.

Chart choice rule

Sequence → process charts; path of movement → flow or string diagram; timing of several subjects → multiple activity chart; hand motions → SIMO chart

Use this to pick the tool in any question.

Machine utilisation on activity chart

Utilisation % = Machine working time ÷ Cycle time × 100

Applies to a multiple activity chart where the cycle time is the same for every subject.

Therblig groups

Therbligs = Effective (productive) + Ineffective (non-productive)

Effective: reach, grasp, move, position, pre-position, assemble, use, disassemble, inspect, release load. Ineffective: search, select, plan, hold, rest, avoidable delay. Aim: shorten the first, eliminate the second.

Principles of motion economy: three heads

Human body + Workplace layout + Tools and equipment

Write every principle under one of these three heads in an answer.

Human body principles

Both hands start and end together; arms move in opposite, symmetrical directions; hands not idle except in rest

Use the lowest classification of movement that does the job (fingers, then wrist, forearm, upper arm, shoulder, body). Smooth curved motions are better than sharp straight-line changes of direction.

Workplace principles

Fixed place for tools and materials; tools and materials pre-positioned and within normal working area

Use gravity feed bins and drop delivery, proper lighting and a chair of the right height.

Tools and equipment principles

Combine tools; pre-position tools; share load among fingers by their strength

Use jigs and fixtures to hold work; use foot-operated devices to free the hands.

Basic time (normal time)

Basic time = Observed time × Rating ÷ Standard rating

If standard rating is 100, basic time = observed time × rating ÷ 100. Rating above standard increases basic time.

Standard time

Standard time = Basic time + Allowances

If allowance is a percentage of basic time: Standard time = Basic time × (1 + allowance %).

Allowance as percentage of total (gross) time

Standard time = Basic time ÷ (1 − allowance % of standard time)

Use this only when the question states the allowance as a percentage of the standard (total) time.

Number of observations (sample size)

Number of observations is chosen from the variation seen in a trial set of readings

More variable elements need more readings. Use only the method given in the question.

Work sampling: percentage of activity

Activity % = Observations of the activity ÷ Total observations × 100

Time for the activity = Activity % × total time period studied.

Standard output

Standard output per period = Available time ÷ Standard time per unit

Use consistent units, such as minutes per unit and minutes per shift.

Normal time (basic time rated)

Normal time = Observed time × (Rating ÷ 100)

Rating is on a scale where 100 is normal pace. Apply it to each element's time.

Standard time with allowance as % of normal time

Standard time = Normal time × (1 + Allowance %)

Use when the question says allowance is a percentage of normal time.

Standard time with allowance as % of standard time

Standard time = Normal time ÷ (1 − Allowance %)

Use when allowance is stated as a percentage of the total (standard) time, or of the working day.

Average observed time

Average observed time = Σ observations ÷ number of observations

Do this first if several readings are given for an element. Exclude abnormal readings if the question tells you to.

Standard time of a job

Job standard time = Σ standard time of all elements

Add elements after rating and allowances. Add any contingency or fatigue allowance as the question states.

Quick revision

• Work study = method study + work measurement.
• Method study aims to find a better, simpler and cheaper way of doing a job.
• Method study steps run from selecting the job to installing and maintaining the new method.
• Recording tools include outline process chart, flow process chart, flow diagram and two-handed chart.
• Motion study examines body movements to remove unnecessary effort.
• Principles of motion economy cover the body, the workplace and tools and equipment.
• Time study is a work measurement technique that uses a stopwatch on a sample of the work.
• Basic time = observed time × rating ÷ 100.
• Standard time = basic time + allowances.
• Allowances cover personal needs, fatigue and unavoidable delays.
• Rating is applied to observed time to get basic time, not to allowances.
• Allowances are normally a percentage of basic time. If the question gives them as a percentage of standard (total) time, use standard time = basic time ÷ (1 − allowance %). Read the question for the base.

Common mistakes

• Treating work study and method study as the same thing. Fix: Remember that method study is only one half. Work study also includes work measurement.
• Saying work measurement comes before method study. Fix: Improve the method first, then set the standard time for the improved method.
• Writing the steps in the wrong order, for example examine before record. Fix: Remember that you cannot judge a method before you have recorded it. Practise the sequence until it is automatic.
• Mixing method study with time study. Fix: Method study decides how to do the job. Time study or work measurement decides how long it should take. Do not put rating or allowances in a method study answer.
• Showing transport, delay and storage in an operation process chart. Fix: Remember that the operation process chart has only operations and inspections. Flow process chart adds the other three.
• Calling a flow diagram and a string diagram the same thing. Fix: A flow diagram draws the path as lines with symbols. A string diagram uses thread to measure the distance and frequency of movement.
• Treating motion study and time study as the same thing. Fix: Motion study improves the method by removing wasteful movements. Time study measures the time for a job. Motion study is part of method study; time study is part of work measurement.
• Calling hold or rest an effective therblig. Fix: Remember that hold, rest, search, select, plan and avoidable delay do not move the job forward. Treat them as ineffective and try to eliminate or reduce them.
• Treating observed time as standard time. Fix: Always write the chain: observed time, basic time, standard time.
• Applying rating in the wrong direction. Fix: A worker faster than standard has a rating above 100, so basic time is higher than observed time.

Exam tips

• Always mention both method study and work measurement, even if the question only says 'work study'.
• In 'distinguish' questions, use a two-column comparison in points: purpose, focus, output, tools. Keep it in plain lines if space is short.
• For MCQs, 'standard time' points to work measurement and 'best method' points to method study.
• Add one practical example from a factory or an office to lift a theory answer.
• Write the procedure steps in order, with one line of purpose each.
• Write the seven steps as a numbered list with short explanations. Examiners look for the sequence and for a line on each step.
• In case questions, tie each step to the facts given, such as layout, distance or number of operators.
• Show a small calculation of saving if figures are given, using the old and new method values.