Work Sampling2

8/14/2019 Work Sampling2

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WORK SAMPLING ANDWORK SAMPLING ANDSTRUCTURED ESTIMATINGSTRUCTURED ESTIMATING

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MEGHANT NICHANTMEGHANT NICHANTINDUSTRIAL ENGINEER - IEINDUSTRIAL ENGINEER - IE


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Work samplingWork sa

mpling is a method of finding the percentage occurrence of ais a method of finding the percentage occurrence of acertain activity by statistical sampling and random observationscertain activity by statistical sampling and random observations..

Work sampling, or activity analysis, is the process of making sufficientWork sampling, or activity analysis, is the process of making sufficientrandom observations of an operators activities to determine the relativerandom observations of an operators activities to determine the relativeamount of time the operator spends on the various activities associatedamount of time the operator spends on the various activities associatedwith the job.with the job.

The major goal of work sampling is to determine how long, or how much ofThe major goal of work sampling is to determine how long, or how much ofthe work day, is spent on specific types of work.the work day, is spent on specific types of work.

Work sampling may identify the fact that certain operators spend a largeWork sampling may identify the fact that certain operators spend a largeportion of their time waiting for work, or performing paperwork tasks, orportion of their time waiting for work, or performing paperwork tasks, oreven performing activities that are not included in their job descriptions.even performing activities that are not included in their job descriptions.

One of the basic foundations of statistical sampling theory is the conceptOne of the basic foundations of statistical sampling theory is the conceptthat the larger the sample size, the results will be better or more accurate.that the larger the sample size, the results will be better or more accurate.

In work sampling, a sufficient number of observations must be made to beIn work sampling, a sufficient number of observations must be made to besure that the results accurately summarize the work performed. There aresure that the results accurately summarize the work performed. There arestatistical formulas to help determine how many observations should bestatistical formulas to help determine how many observations should bemade.made.

The number of observations that an analyst must make of a particular jobThe number of observations that an analyst must make of a particular jobalso depends on how much time is devoted to a particular task.also depends on how much time is devoted to a particular task.


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CONDUCTING A STUDYCONDUCTING A STUDY It is recommended that a uniform procedure should be followed to perform aIt is recommended that a uniform procedure should be followed to perform a

work sampling study is towork sampling study is to

1. Establish the Purpose1. Establish the Purpose First, the objective of the study should be established. Work sampling can beFirst, the objective of the study should be established. Work sampling can be

used to determine an overall perspective on the work doneused to determine an overall perspective on the work done..

2. Identify the Subjects2. Identify the Subjects Second, the people performing the task must be identified, i.e. general officeSecond, the people performing the task must be identified, i.e. general office

work is being studied with the objective of determining overall productivity.work is being studied with the objective of determining overall productivity.

3. Identify the Measure of Output3. Identify the Measure of Output The third step in making the study is the identification of the measure of theThe third step in making the study is the identification of the measure of the

output produced or the types of activities performed on the jobs being studied.output produced or the types of activities performed on the jobs being studied.This step is especially important if the objective of the study is to measureThis step is especially important if the objective of the study is to measureproductivity with the intent of setting a standard.productivity with the intent of setting a standard.

4. Establish a Time Period4. Establish a Time Period Fourth, the time period during which the study will be conducted must beFourth, the time period during which the study will be conducted must be

established. Starting and stopping points for the study must be defined as well.established. Starting and stopping points for the study must be defined as well.

5. Define the Activities5. Define the Activities This step involves defining the activities that are performed by the people underThis step involves defining the activities that are performed by the people under

study. For example, the definition used in a machine utilization study, includingstudy. For example, the definition used in a machine utilization study, including

only the categories of working, idle, and idle-mechanical breakdown.only the categories of working, idle, and idle-mechanical breakdown.


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6. Determine the Number of Observations Needed6. Determine the Number of Observations Needed

After the work elements are defined, the number of observations for the desired accuracy atAfter the work elements are defined, the number of observations for the desired accuracy atthe desired confidence level must be determined. The sample size is dependent on thethe desired confidence level must be determined. The sample size is dependent on thepercentage of time believed to be spent on the major work element.percentage of time believed to be spent on the major work element.

If a reasonable guess cannot be made, then a trial study of perhaps 20 to 40 observationsIf a reasonable guess cannot be made, then a trial study of perhaps 20 to 40 observationsshould be made to get an estimate of this portion. These initial observations should beshould be made to get an estimate of this portion. These initial observations should be

included with the rest of the observations taken during the rest of the work study.included with the rest of the observations taken during the rest of the work study.

7. Schedule the Observations7. Schedule the Observations

Once the number of required observations has been determined, either from appropriateOnce the number of required observations has been determined, either from appropriatestatistical calculations or from tables, the actual observations must be scheduled. Typically,statistical calculations or from tables, the actual observations must be scheduled. Typically,the analyst will assign an equal number of observations each day during the course of thethe analyst will assign an equal number of observations each day during the course of thestudy.study.

For example, if 800 observations are required and 20 work days are established as anFor example, if 800 observations are required and 20 work days are established as an

appropriate observation time, 40 observations should be recorded each day.appropriate observation time, 40 observations should be recorded each day. A random number table can be used to establish the random times for each observation.A random number table can be used to establish the random times for each observation.

8. Inform the Personnel Involved8. Inform the Personnel Involved

Before the study is actually performed, the personnel involved should be informed about theBefore the study is actually performed, the personnel involved should be informed about theobjective of the study and the methodology that will be employed. As in any productivityobjective of the study and the methodology that will be employed. As in any productivitymeasurement study, this part of the procedure is very important.measurement study, this part of the procedure is very important.

Workers and their supervisors might think that they personally are being measured ratherWorkers and their supervisors might think that they personally are being measured rather

than the work they are doing.than the work they are doing.

9. Record the Raw Data9. Record the Raw Data

The next and perhaps the easiest part of any work sampling study is the actual recording ofThe next and perhaps the easiest part of any work sampling study is the actual recording ofthe raw data. Although this recording can be performed by anyone, it is desirable that athe raw data. Although this recording can be performed by anyone, it is desirable that atrained analyst be employed.trained analyst be employed.

It is also very important that the observations be made at exactly the same location everyIt is also very important that the observations be made at exactly the same location everytime. Failure to be reliable in this manner may bias the resultstime. Failure to be reliable in this manner may bias the results..

10. Summarize the Data10. Summarize the Data After the data have been collected, they must be summarized.After the data have been collected, they must be summarized.


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A few words about samplingA few words about sampling

Sampling is mainly based on probability. Probability has been definedSampling is mainly based on probability. Probability has been definedas as the degree to which an event is likely to occurthe

degree to which an event is likely to occur..

A simple and often-mentioned example that illustrates the point is thatA simple and often-mentioned example that illustrates the point is thatof tossing a coin.of tossing a coin.

The law of probability says that we are likely to have 50 heads and 50The law of probability says that we are likely to have 50 heads and 50tails in every 100 tosses of the coin. The greater the number of tosses,tails in every 100 tosses of the coin. The greater the number of tosses,the more chance we have of arriving at a ratio of 50 heads to 50 tails.the more chance we have of arriving at a ratio of 50 heads to 50 tails.

The size of the sample is therefore important, and we can express ourThe size of the sample is therefore important, and we can express ourconfidence in whether or not the sample is representative by using aconfidence in whether or not the sample is representative by using acertain confidence levelcertain confidence level..

Establishing confidence levelsEstablishing confidence levels

Let us go back to our previous example and toss five coins at a time,Let us go back to our previous example and toss five coins at a time,

and then record the number of times we have heads and the numberand then record the number of times we have heads and the numberof times we have tails for each toss of these five coins. Let us thenof times we have tails for each toss of these five coins. Let us thenrepeat this operation 100 times.repeat this operation 100 times.

If we considerably increase the number of tosses and in each caseIf we considerably increase the number of tosses and in each casetoss a large number of coins at a time, we can obtain a smoothertoss a large number of coins at a time, we can obtain a smoothercurve, such as that shown in figure 89curve, such as that shown in figure 89..


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To make things easier, it is more convenient to speak of a 95 per cent confidenceTo make things easier, it is more convenient to speak of a 95 per cent confidencelevel than of a 95.45 per cent confidence level.level than of a 95.45 per cent confidence level.

To achieve this we can change our calculations and obtainTo achieve this we can change our calculations and obtain::

95 per cent confidence level or 95 per cent of the area under the curve = 1.9695 per cent confidence level or 95 per cent of the area under the curve = 1.96 99 per cent confidence level or 99 per cent of the area under the curve = 299 per cent confidence level or 99 per cent of the area under the curve = 2..5858 99.9 per cent confidence level or 99.9 per cent of the area under the curve = 3.399.9 per cent confidence level or 99.9 per cent of the area under the curve = 3.3

In this case we can say that if we take a large sample at random we can be confidentIn this case we can say that if we take a large sample at random we can be confidentthat in 95 per cent of the cases our observations will fall within 1.96that in 95 per cent of the cases our observations will fall within 1.96


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Determination of sample sizeDetermination of sample size

As well as defining the confidence level for our observations we haveAs well as defining the confidence level for our observations we haveto decide on the margin of error that we can allow for theseto decide on the margin of error that we can allow for theseobservations.observations.

Let us look at our example about the productive time and the idle timeLet us look at our example about the productive time and the idle timeof the machines in a factory. There are two methods of determiningof the machines in a factory. There are two methods of determiningthe sample size that would be appropriate for this example:the sample size that would be appropriate for this example:

thethe statistical methodstatistical method and theand the nomogram methodnomogram method..

Statistical methodStatistical method. The formula used in this method is. The formula used in this method is::


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Let us assume thatLet us assume that some 100 observationssome 100 observations were carried out as a preliminarywere carried out as a preliminarystudy and at random, and that these showed the machine to be idle in 25 perstudy and at random, and that these showed the machine to be idle in 25 percent of the cases (p = 25) and to be working 75 per cent of the time (q = 75).cent of the cases (p = 25) and to be working 75 per cent of the time (q = 75).

We thus have approximate values for p and q; in order now to determine theWe thus have approximate values for p and q; in order now to determine thevalue of n, we must find out the value of .value of n, we must find out the value of .

Let us choose a confidence level of 95 per cent with a 10 per cent margin ofLet us choose a confidence level of 95 per cent with a 10 per cent margin of

error (that is, we are confident that in 95 per cent of the cases our estimates willerror (that is, we are confident that in 95 per cent of the cases our estimates willbe 10 per cent of the real value).be 10 per cent of the real value).


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Nomogram methodNomogram method

An easier way to determine sample size is to read off the number of observationsAn easier way to determine sample size is to read off the number of observations

needed directly from a nomogram such as the one reproduced in figure 91.needed directly from a nomogram such as the one reproduced in figure 91.


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Making random observationsMaking random observations

To ensure that our observations are in fact made at random, we canTo ensure that our observations are in fact made at random, we canuse a random table such as the one in table 12.use a random table such as the one in table 12.

Various types of random table exist, and these can be used inVarious types of random table exist, and these can be used indifferent ways. In our case let us assume that we shall carry out ourdifferent ways. In our case let us assume that we shall carry out ourobservations during a day shift of eight hours, from 7 a.m. to 3 p.m.observations during a day shift of eight hours, from 7 a.m. to 3 p.m.An eight-hour day has 480 minutes. These may be divided into 48An eight-hour day has 480 minutes. These may be divided into 48ten-minute periodsten-minute periods..

We can start by choosing any number at random from our table, forWe can start by choosing any number at random from our table, forexample by closing our eyes and placing a pencil point somewhereexample by closing our eyes and placing a pencil point somewhereon the table. Let us assume that in this case we pick, by simpleon the table. Let us assume that in this case we pick, by simplechance, the number 11 which is in the second block, fourth column,chance, the number 11 which is in the second block, fourth column,fourth row (table 12).fourth row (table 12).

We now choose any number between 1 and 10. Assume that weWe now choose any number between 1 and 10. Assume that wechoose the number 2; we now go down the column picking out everychoose the number 2; we now go down the column picking out everysecond reading and noting it down, as shown below (if we hadsecond reading and noting it down, as shown below (if we hadchosen the number 3, we should pick out every third figure, and sochosen the number 3, we should pick out every third figure, and soon).on).

1111 38 45 87 68 20 11 26 49 0538 45 87 68 20 11 26 49 05


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Looking at these numbers, we find that we have to discard 87, 68 and 49Looking at these numbers, we find that we have to discard 87, 68 and 49because they are too high (since we have only 48 ten-minute periods, anybecause they are too high (since we have only 48 ten-minute periods, anynumber above 48 has to be discarded).number above 48 has to be discarded).

Similarly, the second 11 will also have to be discarded since it is a number thatSimilarly, the second 11 will also have to be discarded since it is a number thathas already been picked out. We therefore have to continue with our readingshas already been picked out. We therefore have to continue with our readingsto replace the four numbers we have discarded. Using the same method, that isto replace the four numbers we have discarded. Using the same method, that is

choosing every second number after the last one (05), we now havechoosing every second number after the last one (05), we now have1414 15 4715 472222

These four numbers are within the desired range and have not appeared before.These four numbers are within the desired range and have not appeared before.Our final selection may now be arranged numerically and the times ofOur final selection may now be arranged numerically and the times ofobservation throughout the eight-hour day worked out. Thus our smallestobservation throughout the eight-hour day worked out. Thus our smallestnumber (05) represents the fifth ten-minute period after the work began at 7number (05) represents the fifth ten-minute period after the work began at 7a.m. Thus our first observation will be at 7.50 a.m., and so on (table 13).a.m. Thus our first observation will be at 7.50 a.m., and so on (table 13).


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Example: Conducting the studyExample: Conducting the study

Determining the scope of the study. Before making our actual observations, it isDetermining the scope of the study. Before making our actual observations, it is

important that we decide on theimportant that we decide on the objectiveobjective of our work sampling.of our work sampling.

The simplest objective is that of determining whether a given machine is idle orThe simplest objective is that of determining whether a given machine is idle or

working.working.

In such a case, our observations aim at detecting one of two possibilities onlyIn such a case, our observations aim at detecting one of two possibilities only::

We can, however, extend this simple model to try to find out the cause of theWe can, however, extend this simple model to try to find out the cause of thestoppage of the machinestoppage of the machine::


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Making the observationsMaking the observations

So far we have taken the first five logical steps in conducting aSo far we have taken the first five logical steps in conducting awork sampling study.work sampling study.

selecting the job to be studied and determining the objectives ofselecting the job to be studied and determining the objectives ofthe study;the study;

making a preliminary observation to determine the approximatemaking a preliminary observation to determine the approximate

values of p and q;values of p and q;

in terms of a chosen confidence level and accuracy range,in terms of a chosen confidence level and accuracy range,determining n (the number of observations needed) determiningdetermining n (the number of observations needed) determiningthe frequency of observations, using random tables;the frequency of observations, using random tables;

designing record sheets to meet the objectives of the studydesigning record sheets to meet the objectives of the study..

There is one more step to take: that of making and recording theThere is one more step to take: that of making and recording theobservations and analyzing the results.observations and analyzing the results.


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Group sampling techniquesGroup sampling techniques As the name suggests, these are designed for the measurement ofAs the name suggests, these are designed for the measurement of

work carried out by groups of workers.work carried out by groups of workers. The techniques are sometimes referred to by the term The techniques are sometimes referred to by the term high-frequencyhigh-frequency

samplingsampling since, when used for the measurement of short-cycle work, since, when used for the measurement of short-cycle work,they use fixed short-time intervals with the observer in constantthey use fixed short-time intervals with the observer in constantattendance.attendance.

They are very close to time study but have the advantage that theThey are very close to time study but have the advantage that theobserver can cover the work of the group. Group sampling techniquesobserver can cover the work of the group. Group sampling techniquesmay make use of ratingmay make use of rating..

Consider a very simpleConsider a very simple example of three workersexample of three workers each producing theeach producing thesame parts by a process that involves only hand tools. The samplingsame parts by a process that involves only hand tools. The samplingis carried out atis carried out at 0.5 minute intervals0.5 minute intervals and involves the categories ofand involves the categories of

working and not working only.working and not working only. The sampling observations have been rated and this is an example ofThe sampling observations have been rated and this is an example of

both rated activity sampling and group samplingboth rated activity sampling and group sampling..

The sampling sheet would look as shown in table 14The sampling sheet would look as shown in table 14..


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Work Sampling2

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