Work Measurement technique

A Survey of Work Measurement Techniques

    Dr. W. A. Woeber, PrEng, DSc(Tech), FIMechE, FIProdE

Seventy-five years have passed since Frederick Winslow Taylor presented his Paper on "A Piece-Rate System" to the American Society of Mechanical Engineers. It was he who adopted the term time-study and who set the pattern for the study of time elements and cycle times. Today, in operational research, we feel that indirect measures of fact are preferable to direct operational measures with the result that statistical and mathematical techniques have, in many instances, replaced the basic time study technique.

Although stopwatch time study and elemental time values are useful for measuring repetitive short cycle labour operations, most indirect activities are characterised by relatively long, irregular cycles requiring a work measurement technique based on statistical principles that allow generalisations about the entire time span to be made on the basis of a sample of that time interval.

Rating research:

                        The object of modern time study is to assess the work content of a specific task in terms of the time it should take a fully trained and experienced worker to carry out that task at "standard performance". This is achieved by comparing the performance of the worker with the observer's own concept of standard performance which is rated at 100 on the B.S. Scale. In addition to assessing the worker's rate of working, the time taken to complete the task is measured.

                 By multiplying the rating by the time taken, the Basic Minute Value (BMV) of the task is obtained. Certain relaxation allowances, which take into account the ergonomic and environmental conditions under which the task is performed, are added to the BMV to derive the Standard Minute Value (SMV).

                                                                             The random variations in performance times of a particular worker may be assumed to be normally distributed. The distribution will have a mean y. and a standard deviation a. The standard error will be equal to a divided by \/n, where n, the number of observations, represents the sample size. The actual size of the sample will be determined by the degree of accuracy which the analysis demands. To assess a "qualified" worker's rate of working raises the question of how the attributes of such a worker are to be defined.

                                              In the Westinghouse System the worker is evaluated in terms of four factors. These are skill, effort, consistency, and working conditions. Briefly, skill is defined as the proficiency at following a given method; effort, as the will to work; consistency, as the degree of variation in performance times; and conditions, as the characteristics of the physical environment which affect the worker, such as noise, light, heat and humidity.

                               Firstly, if normal working times (NWT) for the elements are available, a company can develop standard data for future use to describe a new job. Secondly, elemental time data permit making better estimates of the probable effect of method changes.  A variation and extension of effort rating was developed by Mundel5. He claims to have reduced the degree of subjective judgment to what he has called "objective rating". A modified version of the objective rating system has been proposed by Nadler6. Known as "pace rating", it follows the same procedure except that the rating of the worker's operating pace is obtained by selecting a speed, which is comparable, from a "step film". The film loop consists of a series of gradually increasing working paces of the performances of a simple job and each different pace is preceded by a symbol identifying the performance level. The pace rating system, like all the others, has its limitations; difficulties are encountered in selecting typical jobs and , judgment plays an inordinate role in their selection. An interesting research project was undertaken by Desmond who developed a method of analysis to measure three major defects in the techniques of work measurement, namely errors in the concept of normal performance, "flatness" of rating, or the inability to appreciate proportionate changes in speed, and residual inconsistency of rating, or the inability to recognize the same speed when seen on more than one occasion. A simplified graphical version of Desmond's "reciprate method", based on regression analysis, is shown in Fig. 1.

The ratings are plotted against the times on specially prepared graph paper, in which the rating scale is proportional to the reciprocal of the rating. A rule is then placed through the origin and rotated until it produces the best fit. The time corresponding to the intersection of this line with the normal performance line is then the estimated normal time. The next step consists of drawing the study line by eye and measuring, on a linear scale, the height of its intersection with the original line through the origin. This distance is represented by the symbol E in Fig. 1. The height of the intersection of the study line with the rating axis is also measured and it is represented by the symbol D; this can be a negative quantity when the particular study is steep.

The ratio D/E is then the flatness of the study, and records of variation from study to study give a good indication of the ability to maintain a certain quality of rating with respect to this defect. Such records are conveniently kept as control charts, although calculation of control limits is subject to considerable difficulty.

                         The inconsistency of the study is determined graphically by means of the range of scatter about the estimated operation line. The plotted point is chosen which lies at a maximum vertical distance above the line, and this distance is represented by the symbol A. Similarly, the distance B represents the semi-range below the estimated operation line. Then the sum of these distances A + B = to is the range of reciprate scatter and can be used to estimate the standard deviation about the line. Desmond concluded his paper by stating that the method had been applied to the analysis of some 32 000 observations which had been collected during a nation-wide survey of time study rating accuracy.

Work sampling techniques:

                                   Work sampling is based upon statistical principles that allow generalisations about the entire time span to be made on the basis of a sample of that time interval. The technique, also described as the ratio-delay method, originated with L H. C. Tippett10of the Shirley Institute of the British Cotton Industry Research Association. The fundamental principle of work sampling may be stated thus: the number of observations is proportional to the amount of time spent in the working or idle state. The accuracy of the estimate depends on the number of random observations and on pre-set precision limits and confidence levels. The treatment of the data requires the setting up of a statistical model following a binomial distribution; statistical quality control methods can be used in analysis. Hence, from the simple formula for mean proportion:

and tables have been developed which give control limits. (Barnes). See Fig.

Quantitative approaches to the solution can be made by assuming that the nature of the service and arrival times follows a Poisson distribution. Essential to waiting-time analysis is the ratio of service time to between-service time. This ratio is defined as K. Work sampling provides a convenient means for the collection of the data.

If (in fractions)

m = machine time,

b = service time and

d = machine idle time,

then

m + b + d = 1

and

K = b/m

Once K is known, tables, such as those compiled by Docent Conny Palm, may be consulted to find the changes in the various times of idling, service, and operation with changes in number of machines per serviceman.

Another approach is Monte Carlo simulation. This method calls for estimating on the basis of past experience the probabilities of occurrence to be associated with the various possible service and arrival times.

Physiological work measurement:

                                          In physics and the engineering sciences "work" is defined as the scalar product of force and displacement. In

Physiology, however, the concept "work" has a more complex meaning. A physiologist sometimes speaks of "work", where it would be better to use the term "effort". "Energy" is the capacity for doing work. Physical working capacity, or the physiological limit for sustained work, is measured as the worker's maximum oxygen intake. Economy of effort in the performance of a task is expressed as the rate of oxygen in litres per minute, the physiologist's measure of energy expended to the work performed. Energy expenditure data are also stated in terms of calories per minute and heart rate data in beats per minute.

Relaxation allowance:

                             This is defined2 as "an addition to the basic time intended to provide the worker with the opportunity to recover from the physiological and psychological effects of carrying out specified work under specified conditions". To determine allowances for recovery from fatigue, the time study analyst uses tables which are based upon estimates of the physiological strain the worker is likely to experience as a result of the physical effort of the task and any environmental stresses which might add to the physical effort.

Physical effort is estimated from the metabolic activity of the worker, the air exhaled by him being collected in a Douglas bag. This is then quantitatively analysed and the rate of oxygen consumed per unit of time is computed.

Under ideal conditions, if

£ = total energy expenditure per minute,

r = rate of working,

n = energy expenditure per minute due to normal metabolism, and

m = energy expenditure per minute not due to normal metabolism,

then

£ = mr + n,

or

m= (E - n)/r

Summing up, it can be stated that where physical effort is involved in an operation, information can be obtained by means of energy expenditure measurements, the physiological strain involved and the time necessary for circular respiratory recovery.

Conclusion

As a result of increased mechanisation in industry, the muscle load on the worker will be reduced. The man machine system will give him less and less to do with his legs, arms and trunk, and more to do with his eyes and his fingers in the way of watching and manipulating controls, pointers and switches. This is the change of the work load from the more physiological elements to the psychological aspects of man. The physical load is diminished and the perceptual load increased.