Work Sampling

Presented By, Suhrita Ghosh (22)

Susmita Biswas (23)

Definition Explaining Work Sampling Types of Work Sampling Procedure for Work Sampling Normal Distribution Curve Confidence Level Determining Sample Size Errors in Work Sampling Uses of Work Sampling Advantages of Work Sampling Disadvantages of Work Sampling Bibliography

Contents

Work Sampling : A method of finding the percentage occurrence of a certain activity by statistical sampling and random observations.

Introduced by L.H.C. TIPPET of British cotton industry research association in 1934.

Also known as Activity Sampling or Random Sampling.

Definition

Based upon laws of probability.

A sample taken at random from a large group tends to have the same pattern of distribution as the large group.

Sample is the term used for the small representative number.

Population or Universe is the term used for the large group.

Obtaining and analyzing only a part of the universe is called Sampling.

Larger the sample size, the more accurate or representative it becomes with respect to the original “population”.

A panel of 480 blocks representing 480 minutes of a 8 hour day arranged across the bottom of a board.

The white blocks represent working time and the colored idle time.

480 small pieces of paper are taken and numbered to represent 480 minutes in the 8 hour day.

These slips kept in a hat and mixed thoroughly. A number is drawn and a mark placed on a bar at the time

corresponding to the number on the slip. 40 such ‘random observations’ are done.

Explaining Work Sampling

Markings are on white represent observations of the worker while working and those on colored represent him idle.

Total of 6 idle observations were taken during the day. Therefore, (6/40)*100=15 =15% This is the percentage of the day during which the worker

was idle as determined by work sampling.State Tally Total

Working IIII IIII IIII IIII IIII IIII 34

Idle IIII I 6

Activity or Delay Sampling- To measure the activities and delays of workers or machines.

Performance Sampling- To measure working time and nonworking time of a person on a manual task, and to establish a performance index or performance level for the person during his working time.

Work Measurement- Under certain circumstances, to measure a manual task, that is to establish a time standard for an operation.

Types of Work Sampling

The following steps are required in making a work sampling study:-

Define the problem. a. State the main objectives of the problem. b. Describe in detail each element to be measurement.

Obtain the approval of the department in which the work sampling study is to be made. Obtain the co-operation of the workers.

Procedure

Determine the desired accuracy of the final results.

Make a preliminary estimate of the percentage occurrence of the activity or delay to be measured, based on past experience or a one-day or two-day preliminary work sampling study.

Design the study. a. Determine the number of observations to be made. b. Determine the number of observers needed. c. Determine the number of shifts needed. d. Make detailed plans for taking the observations such

as the time and route to be followed by observer. e. Design the observation form.

Make the observation. a. Make the observation and record the data. b. Summarize the data at the end of each day. c. Determine the control limits. d. Plot the data on control charts at the end of

each day.

Check the accuracy or precision of data at the end of the day.

Prepare the report and state conclusions. Make recommendations if called for.

It is necessary to decide what level of confidence is desired in the final working sample.

Most common confidence interval is 95% which means that probability is 95% of the timesthe random observations willrepresent the facts and 5% ofthe time, it will not.

Confidence Level

It represents graphically the probability of the occurrence of certain chance phenomenon.

Normal distribution curve is described by 2 attributes : x=average or measure of central dispersion σ =standard deviation(deviation from average)

Normal Distribution Curve

From the previous figure,σp on both sides of x gives an area of 68.27% of the total area.

2σp on both sides of x gives an area of 95.45%. 32σp on both sides of x gives an area of 99.73%.

This is in fact the degree of confidence in our observations. For e.g.

95% confidence level or 95% of the area under the curve=1.96σp

99% confidence level or 99% of the area under the curve =2.58σp

99.9% confidence level or 99.9% of the area under the curve =3.3σp

The proportion defective items ‘p’ in a large number of items is to be drawn after observing the proportion defective ‘p1’ in a suitably selected sample size ‘n’ from the large number of items.

The standard deviation of ‘p1’ for sample size ‘n’, also termed as Standard Error of Proportion,

given by, σ p = √{p(1-p)} √(n-1)

Determination of sample size

For large ‘n’ the expression can be approximated to:-

σ p = √{p(1-p)} √n

When ‘p’ is unknown, the average value of p1 computed from previous samples maybe used in place of p.

We have to decide the margin of error that we can allow for these observations.

The formula for determining the number of observations is given by :

P.S=Kσp =K√p(1-p) √n

p= percentage occurrence of the activity (working or non-working) being measured in fraction.

s= error in fractionK= a factor, the value of which depends on the desired

confidence level. For e.g. 95% confidence level, K=1.96n= number of observations required for the desired

confidence level and margin of error.

Work Sampling is subjected to two kinds of error.

Observational Error: Occurs due to the presence and behavior of the observer, the observed and/or environment in which the observations are made.

Study is made on a finite period of time. The period may

not be representative.

Error in work sampling

Experimental Error:

Due to finite number of random observations on a specified activity, an experimental error is introduced which is a measure of sampling variations in terms of Standard Error of Proportion.

To aid in determination of time standards and delay allowances.

To aid in the measurement of overall performances.

To determine the nature and extent of cycles and ‘peak load’ variations in observable activity.

To study the time utilization by supervisors and establishing goals for supervision.

Uses of Work Sampling

To aid in job evaluation.

To assist in engineering economy studies.

To aid in manpower planning.

For appraisal of safety performance.

For appraisal of organizational efficiency.

Comparatively simple method.

Usually requires fewer man hours and costs less to make a work sampling study than it does to make a continuous time study.

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.

Work sampling may be interrupted at any time without affecting the result.

Operator does not feel under too much pressure and attention.

Advantages of work sampling

A simultaneous work sampling study of several operation or operators or machines maybe made by a single observer.

Not necessary to use trained analyst for observers.

Prolonged study is unnecessary since instantaneous observation of the operator at random intervals during the working day.

Less tedious and less fatiguing to make on the part of the observer.

A stop watch is not needed. If an electronic data collector is used, the results are shown on a computer printout.

Not economical for studying a simple a single operator or machine or for studying operators or machines located over wide areas.

No details about operation.

• No No details about operationdetails about operation

Disadvantages

Introduction to work study( Indian Adaptation) International Labor Office, Geneva

Motion and time study design and measurement of work(Seventh Edition)

-Ralph M. Barnes

Bibliography

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