Work Sampling

Work sampling or activity sampling is a method of finding the percentage occurrence of certain activity by statistical sampling and random observations. Work sampling utilises the principle of drawing inferences from a random sample of the whole. It is an extremely useful work measurement technique as it is inexpensive, convenient, reliable and it can be done without using the stop watch or any subjective judgements of effort or rating (rate of performance).

Work sampling is particularly useful in the analysis of non-repetitive or irregularly occurring activities.

To determine the percentage of the working day that a worker or a machine is working or idle, the observer visits the workplace for predetermined number of times per day at random. The observations are recorded as follows : Fig. 1.13.

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In the chart Fig. 1.13 the total number of observations made are 40, out of which 32 times the worker was found to be working and 8 times the worker was idle. The conclusion drawn is that 80 percent of the time 32 × 100

40

 

 

  the worker is working and 20 percent of the time 8 × 100 40       the worker is idle.

Steps in Making Work Sampling Study

The following steps are involved in making work sampling study : 1. Define the problem

(a) State the main objective or purpose of the study. (b) Describe the details of each element to be measured.

2. Obtain the approval of the incharge of the department / section in which the work sampling study is to be made. Make certain that the operators to be studied and other people in the department understand the purpose of the study and they should co-operate in the study. 3. Design the study

(a) Determine the number of : (i) Observations to be made,

(ii) Observers needed (if more than one observer is required). Select and instruct these people,

(iii) Days or shifts needed for the study.

There is a scientific procedure to determine the number of observations to be made, depending on the accuracy required. However, for practical purposes 40 to 50 observations spanning the entire period of activity of the concerned shop, office or plant are required. While determining the span or cycle of the period the peak periods and lean periods of the activity should be taken into consideration.

(b) Make plans for taking the observations such as time and route to be followed by the observer. The visits to the shop for taking observations should be done randomly w.r.t. time of visit and route to be followed.

(c) Design the observation form. Make the observations and record the data. 4. Summarize the data at the end of each day and at the end of the study period. 5. Make recommendations, if required.

Fig. 1.13. Work sampling observations

Observation as to Tally (No. of % working or idle observations)

Working | | | | | | | | | | | | | | | |

| |

| | | | | | | | (32) 80% Idle | | | | | | | . (8) 20%

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Applications of Work Sampling

1. Work sampling is applied in the estimation of the percentage utilisation of machine tools and other equipment, workers etc.

2. It can be used to estimate the time consumed by various activities, i.e., operation, supervision, repair, inspection etc.

3. Work sampling is also used to find out time standards, specially where job is not of repetitive nature and where time study by stop watch method is not applicable.

Time standards can also be calculated by the following formula :

Standard time = Normal time + Allowances (expressed as a percentage of normal time)

=

Total time Working time Performance index

× ×

in minutes in percentage (rating) in percentage

_{Total number of items produced} + Allowances

Advantages and Disadvantages of Work Sampling Compared to Time Study Advantages

1. It involves less cost as compared to stop watch study.

2. Many operations or activities which are difficult and costly to determine by time study can be readily estimated by work sampling.

3. It is possible to simultaneously study a group of operators and activities by a single work study engineer.

4. It does not involve any timing device like stop watch.

5. Observations may be taken over a period of days or weeks, thus decreasing the chance of day-to-day or week-to-week variations affecting the results.

6. If the study gets interrupted in between it does not introduce any error in the results. 7. Operator is not subjected to long periods of stop watch observations.

8. The random observations are taken thus avoiding prolonged observations. 9. This produces less fatigue and is less tedious to the work study engineer. Disadvantages

1. It is not economical by this method to study a single operator or machine.

2. It is not economical by this method to study operators or machines located over wide areas. 3. This method does not break the job into elements and thus does not provide elemental

details.

4. Workers may not be able to understand the principles of work sampling as easily as they do time study.

5. It normally does not account for the pace at which an operator is working.

6. No record of study of an individual operator is kept, therefore a new study must be made whenever a change occurs in any element in the method used.

          

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The advantage of analytical estimating is that it extends the scope of work measurement to deal with jobs for which other techniques cannot be used. Standard times are available before the jobs are actually performed or undertaken. Possession of time standards by the industry for jobs like repair and service, plumbing, tool room work etc., will result in improved planning, scheduling, labour control and provide the means for designing and operating a sound financial incentive scheme.

4. Synthesis

Many operations can be conveniently synthesised (totalled up) from elemental times for which time study values have already been collected and are available. The time allowed for a job can, then, be quickly and economically determined by summing up the appropriate elemental times.

Elemental times are also called standard data, elemental data or synthetic time units or values. These should be compiled from very carefully chosen basic elements. All elemental times should be accompanied by the description of method, equipment used and end points of each element should also be defined.