13.2 Tools and Techniques of Work Study
13.2.2 Work Measurement (Time Study)
Methods study and Work measurement are subsets of methods engineering. Time has always been one of the most important variables in engineering and science, including manufacturing. Galileo’s experi- ments with falling bodies, for example, depended greatly on measurements of distance and time. It was
Taylor who offered the concept of measuring the time of human activity as a means of monitoring labor performance in industry.
Time study is a structured process of directly observing and measuring (using a timing device) human work in order to establish the time required for completion of the work by a qualified worker when working at a defined level of performance.
13.2.2.1 Stopwatch Time-Study
This was the first work measurement technique developed by Taylor. In this, the analyst breaks down an operation into elements. Then an operator performs the operation a number of times while the analyst observes elapsed time at the end of each element for a number of cycles of the study. The analyst also observes the rate of activity of the operator and records a ‘performance rating factor’, which is the observed pace of the operator compared to the analyst’s concept of normal pace for the operation under study, considering applicable allowances for the operation.
The average time over several cycles is computed and adjusted for the speed and skill, or per- formance rating (PR), of the worker studied. Finally, an allowance factor (AF) is applied for personal needs, unavoidable delays, and fatigue. It typically represents a proportion of the normal time.
Steps in Conducting a Time Study
1. Select the job, inform the workers, and define the best method.
2. Time an appropriate number of cycles n. Use a sample size chart or graph to determine n.
3. Compute the sample size required.
For desired level of accuracy, sufficient no of observations are required. For many kinds of measurements, a level of ± 5% accuracy is considered satisfactory. For this level of accuracy, the formula for determining the no of observations is
n = Z2 p p h (1 )
2
− where n = required sample size
Z = standard normal deviate for the desired confidence level = 1 for 68 % confidence level
= 2 for 95.45 % confidence level, and = 3 for 99.73 % confidence level
p = estimated value of sample proportion (of time the worker is observed busy or idle) h = acceptable error level, in percent
4. Prepare a schedule to observe the worker at appropriate time. The concept of random numbers may be used to provide for random observations.
5. Observe, rate, and record the worker activities per schedule.
6. Record starting time, stopping time, and no. of acceptable units completed during the period.
7. Compute the normal time (NT)
NT total time % working PR number of units completed
= × ×
8. Compute the standard time (ST)
ST = NT + Allowances
Uses of Time Estimates and Work Standards
• Evaluation of present or past performance
• Worker payment/incentive payment, performance evaluation/productivity measurement
• Equipment and facility evaluation
• Evaluation of alternative work methods and operating procedures
• Prediction of future performance.
It follows the following steps:
• Analysis (i.e., breaking of the work into small, easily-measurable elements);
• Measurement (of these components); and
• Synthesis (from those measured components to arrive at a time for the complete job).
The observer first undertakes preliminary observation of the work (a pilot study) to identify suitable elements which can be clearly recognized, and are convenient, in terms of their length, for measurement.
Subsequent studies are taken during which the observer times each occurrence of each element (using a stopwatch or other timing device) while at the same time making an assessment of the work- er’s rate of working on an agreed rating scale. (One of the prime reasons for measuring elements of work, rather than the work as a whole is to facilitate the process of rating. The rate at which a worker works will vary over time; if elements are carefully selected, the rate of working should be consistent for the relatively short duration of the element. More information on rating is given little later in work measurement.) This assessment of rating is used to convert the observed time for the element into a normal or basic time - a process referred to as ‘extension’. It is essential that a time study observer has been properly trained in the technique and especially in rating.
Time study involves the use of specific control mechanisms to ensure that timing errors are within acceptable limits. Increasingly, timing is done by electronic devices rather than by mechanical stopwatch; some of these devices also assist in converting observed times into normal times.
Number of Observations
The number of cycles that should be observed depends on the variability in the work and the level of accuracy required. Since time study is essentially a sampling technique in which the value of the time required for the job is based on the observed times for a sample of observations, it is possible using statistical techniques to estimate the number of observations required under specific conditions. This total number of observations should be taken over a range of conditions (where these are variable) and, where possible, on a range of workers.
Once a normal time for each element has been determined, allowances are added (for example, to allow the worker to recover from the physical and mental effects of carrying out the work) to derive a standard time.
Time study is a very flexible technique, suitable for a wide range of work performed under a wide range of conditions, although it is difficult to time jobs with very short cycle times (of a few seconds). Because it is a direct observation technique, it takes account of specific and special condi- tions but it does rely on the use of the subjective process of rating. However, if properly carried out it produces consistent results and it is widely used. Additionally, the use of electronic data capture de- vices and personal computers for analysis makes it much more cost effective than previously.
Techniques of Time Study
There are various ways in which work may be measured and a variety of techniques have been estab- lished. The basic procedure, irrespective of the particular measurement technique being used, consists of three stages:
• an analysis phase in which the job is divided into convenient, discrete components, com- monly known as elements;
• a measurement phase in which the specific measurement technique is used to establish the time required (by a qualified worker working at a defined level of performance) to complete each element of work;
• a synthesis phase in which the various elemental times are added, together with appropriate allowances (see below), to construct the standard time for the complete job.
The techniques used to measure work can be classified into those that rely on direct observation of the work, and those that do not. For example, some techniques, such as predetermined motion-time systems(PMTS) and the use of synthetic or standard data can provide times from simulation or even visualization of the work. However, the data on which such techniques are based were almost certainly based on earlier observation of actual work.
Performance Rating
Direct observation techniques (such as time study and analytical estimating) include a process for converting observed times to times for the ‘qualified worker working at a defined level of perform- ance.’ The commonest of these processes is known as rating.
This involves the observer (after appropriate training) making an assessment of the worker’s rate of working relative to the observer’s concept of the rate corresponding to standard rating. This assessment is based on the factors involved in the work - such as effort, dexterity, speed of movement, and consistency. The assessment is made on a rating scale, of which there are three or four in common usage. Thus on the 0-100 scale, the observer makes a judgment of the worker’s rate of working as a percentage of the standard rate of working (100).
The rating is then used to convert the observed time to the normal time using the simple for- mula:
Normal Time = Observed time × Rating factor
Rating is a bit controversial area of measurement since it is a subjective assessment. Where different observers rate differently, the resulting normal times are not comparable. However, practiced rating practitioners are remarkably consistent. It is important that the raters are properly trained, and that this training is regularly updated (to maintain a common perception of standard rating).
Allowances
When carrying out work over a complete shift or working day, workers suffer from the fatigue due to the work undertaken and the conditions under which they are working. The normal practice is to pro- vide some ‘allowance’ to allow the worker to recover from this fatigue and to attend to personal needs.
The amount of allowance depends on the nature of the work and the working environment, and is often assessed using an agreed set of guidelines and scales.
Some suggest that relaxation allowances are unnecessary. With work like carrying of heavy weights, this school suggests that the observer automatically adjusts the concept of standard rating to allow for the weight. Thus, if the standard rate of performance for walking on level ground carrying no weight is equivalent to four miles per hour, then an observer rating a worker walking while carrying a
weight will not expect the equivalent rate. Thus, it is argued that the weight has been allowed for in the adjustment of standard rating and any relaxation allowance is simply a duplication of this adjustment.
In many jobs there are small amounts of work that may occur irregularly and inconsistently. It is often not economic to measure such infrequent work and an additional allowance is added to cover such work and similar irregular delays. This allowance is known as a contingency allowance and is assessed either by observation, by analysis of historical records (for such items as tool sharpening or replacement), or by experience. This result in a standard time which includes the time the work ‘should’
take (when carried out by a qualified worker) plus additional allocations in the form of allowances, where appropriate, to cover relaxation time, contingency time and, perhaps, unoccupied time which increases the overall work cycle (such as waiting for a machine to finish a processing cycle).
13.2.2.2 Other Measurement Techniques
The choice of a suitable measurement technique depends on a number of factors including:
• the purpose of the measurement;
• the level of detail required;
• the time available for the measurement;
• the existence of available predetermined data; and
• the cost of measurement.
To some extent there is a trade off between some of these factors. For example, techniques which derive times quickly may provide less detail and be less suitable for some purposes, such as the establishment of individual performance levels on short-cycle work. These are popular techniques for the Time Study:
(a) Work sampling
(b) Predetermined Motion Time System (PMTS) (c) Synthesis from standard data
(d) Estimating
• Analytical estimating
• Comparative estimating
(a) Work Sampling. This was first introduced in British Textile industry by L. Tippet in 1930s.
It is used to estimate the percent of time that a worker spends on various tasks. It requires random observations to record the activity done by a worker. The results are used to find how employees allocate their time among various activities. This knowledge can lead to changes in staffing, reassign- ment of duties, estimates of activity cost, and the setting of delay allowances for ‘labor standards’.
When work sampling is done to establish delay allowances, it is also called a ‘ratio delay’ study. In a nutshell, work sampling can be used to know the following:
• % of the day a worker is working, and
• % of the day he is idle
• Working and idle time of a worker is used to fix up his performance rating.
• To establish the standard time for an operation.
Advantages Work Sampling
• It involves much less cost as compared to stop watch time study.
• It can be carried out with little training.
• It can time long operations which are almost impractical to be measured by stop watch time study.
• It’s very good for timing group activities.
• It doesn’t need any timing device like stop watch or micro-chronometer, etc.
• Even if the study gets interrupted in between, it doesn’t introduce any error in the results.
• Observations can be made within the desired accuracy.
• Large no. of observations extended over days/weeks damp down the influence of day-to-day fluctuations on the results.
• It can improve efficiency by uncovering the sources of delay.
Limitations:
• It’s uneconomical both as regards time and money to study activities of short duration by work sampling.
• It’s also uneconomical in case one worker/one machine is to be studied.
• It doesn’t break the job into elements and thus doesn’t provide element details.
• It doesn’t help in improving work method.
• It normally doesn’t account for the speed at which an operator is working.
• Workers may not understand the principles of work sampling and hence may not trust it.
Applications:
• To determine working time and idle time of men and machines.
• To time long duration activities which are regular/irregular, frequent/infrequent.
• To estimate the % use of the inspectors and time standard for indirect labor.
• To estimate allowances for unavoidable delay.
• To estimate the time for which material handling equipment are actually operating in a day.
• In describing resource utilization patterns.
• For the purpose of cost control and accounting.
• In stores, hospitals, warehousing, offices, farm work, textile industry, m/c shop, etc.
Steps For Work Sampling
1. Select the job or group to be studied and inform the workers.
2. Take a preliminary sample to obtain an estimate of the parameter value (such as percent of time a worker is busy).
3. Compute the sample size required.
For desired level of accuracy, sufficient no of observations are required. For many kinds of measurements, a level of ± 5% accuracy is considered satisfactory. For this level of accuracy, the formula for determining the no of observations is
n = Z2 p p h (1 )
2
− where n = required sample size
Z = standard normal deviate for the desired confidence level = 1 for 68 % confidence level
= 2 for 95.45 % confidence level, and = 3 for 99.73 % confidence level
p = estimated value of sample proportion (of time the worker is observed busy or idle) h = acceptable error level, in percent
4. Prepare a schedule to observe the worker at appropriate time. The concept of random numbers may be used to provide for random observations.
5. Observe, rate, and record the worker activities per schedule.
6. Record starting time, stopping time, and no. of acceptable units completed during the period.
7. Compute the normal time (NT)
NT total time % working PR number of units completed
= × ×
8. Compute the standard time (ST) 9. ST = NT + Allowances.
Example 13.1. A company estimates that its employees are idle 25 % of the time. It would like to go for work sampling that is accurate within 3 % and wants to have 95.45 % confidence in the results.
Solution: n p p
=Z2 (1h−2 ) where n = required sample size
Z = 2 for 95.45 % confidence level, and p = estimated value of idle proportion = 0.25 h = acceptable error level = 3 % = .03
n p p
=Z2 (1h−2 )
= 4 × .25(1 – .25)/(.03)2 = 833 observations.
If the percent of idle time observed is not close to 25 % as the study progresses, then the no. of observa- tions may have to be recalculated and increased or decreases accordingly.
Example 13.2. Calculate the no of observations necessary for an accuracy of ± 5%, and a confidence level 95% if p = 0.25.
Solution: For 95% confidence level, Z = 1.96
n p p
=Z2 (1h−2 )
= (1.96)2 × .25(1 – .25)/(.05)2 = 288.12 observations.
(b) PMTS (Predetermined Motion Time System). It is a work measurement technique in which operations are divided into fundamental elements and time to complete each element is taken directly from the published Tables. Therefore before doing any operation, its standard time could be known. This is the difference with the time study where standard time (ST) is finalized after the opera- tion is carried out. This is important to various organizations in setting time standards for various tasks.
Advantages of PMTS
••••• The standard can be determined from universally available data.
••••• The standard can be completed before a job is done.
••••• No performance rating is needed.
••••• There is no disruption of normal activities, and
••••• The methods are widely accepted as fair systems of determining standards.
Main uses of PMTS 1. Evaluation of method
••••• To improve the existing methods
••••• To evaluate the proposed methods in advance
••••• To train the operator
••••• To evaluate change in design of tools, jigs, equipment, and the product to determine the standard time.
2. Establishing Time Standards
••••• To get the standard time of any operation by the predetermined data
••••• Comparison could be done with the values obtained by time study
Various Systems of PMTS 1. MTA (Motion Time Analysis) 2. WFS (Work Factor System) 3. BMT (Basic Motion Time-Study) 4. MTM (Methods Time Measurement) 5. 400 System (Western Electric) 6. DMT (Dimension Motion Time)
Time study elements can be quickly analyzed through micro-motion process:
Time Study Element Micro-motion
Take a book from the rack Reach, search, select, grasp
Place a part in a vice Move, pre-position, assemble, release
1. MTA (Motion Time Analysis). A. B. Segur (Oak Park, Illinois) was the first to establish relationship between the time element and the motion itself. From his research, he discovered the law of fundamental times which says, “within reasonable limits, the time required by experts to perform a fundamental motion is a constant.”
He emphasized that the time required to accomplish an act depends on how the work is per- formed (or the method used by the operator).
When an operation is studied, it is generally found that the operation consists of getting some- thing, moving it to some location, processing or assembling it, and then releasing it.
Example 13.3. Operation of writing with a pen might be motion analyzed as follows:
Description (Right Hand) Motion
Move hand to penholder Transport empty Grasp pen in penholder Grasp
Move pen to paper Transport loaded
Write on paper Use
Move pen to penholder Transport loaded Pre-position pen to penholder Pre-position Assemble pen to penholder Assemble
Release pen Release
Move hand back to paper Transport empty
In addition to analyzing the motions involved, certain other information must be collected, such as: the distance moved, the type of grasp, the body members required, and the type of release, etc.
• This information helps the analyst in answering questions like: How was the transport empty, preposition, or grasp performed ? This helps in defining the motions precisely and then choosing the proper time for performing the motions.
2. Work Factor System. In this system, the time for an operation is calculated on the basis of :
• Body member used: six body members are recognized and the time of an element depends on the particular body member used to perform the element. The body members are: finger or head, arm, fore arm swivel, trunk, foot, and head turn.
• Distance moved: the shortest distance between starting and stopping point of a motion or element is considered for time estimate.
• Manual control: can be of following types: definite stop, directional control, carefulness, and change of direction. Each type takes different type. One example could be passing a thread through the needle eye.
• Weight or resistance involved: weight has a positive effect on time. Again the time de- pends on the body member used to perform the element and also on ‘ whether an operation is made by male or a female’.
Example 13.4. If the time taken to move an empty hand through a distance of 10 inch is 0.0042 minute, the time to move a hand with 10 kg weight for the same distance would be 0.005 minute. If a lady does it, she may take 0.006 minute.
Time standards may be established by any one of the following three work-factor systems:
Detailed: when high accuracy is needed.
Simplified: some of the elemental work-factor motions are combined together to get quick measurement of standard time.
Abbreviated WF: where very high precision data is not required. It’s generally applied for non- repetitive work with long time cycles, such as: maintenance operation, material handling operation, m/c set up, and packing, etc.
Work Factor Symbol
Weight W
Directional control S
Precautions P
Definite stop D
3. Methods Time Measurement (MTM). This system is the most common form of PMTS.
This divides any manual operation into basic motions, and then assigns predetermined time value to each. So it could be said that basically it is a technique of method analysis, however, it also does the work of time study by providing time values for each motion. The typical table of MTM system gives time in TMUs (Time Measurement Units). An analyst trained in MTM breaks down an operation into a sequence of elemental activities as classified by the MTM system. The sum the times for these de- tailed activities becomes the basis for establishing the standard time for the operation.