Work Sampling

Work Sampling Origin

Uses of Work Sampling

Basis of Work Sampling

Basis of Work Sampling (cont.)

• Based on an event being present or not.• Probability of x occurrences of an event in n observations: (p +

q)n = 1• Expand IAW binomial theorem, with the 1st term giving the

probability of x=0, the 2nd term x=1, etc…• This distribution of probabilities is known as the binomial

distribution, with the Mean = np, and Variance = npq.• As n becomes large, the binomial distribution approaches the

normal distribution

Example

• Take 5 coins

• Toss each of the coin in one set.

• Count number of time a given combination of head and tail is obtained.

• Combination : 0 No. head and 5 No. of tail.

• Combination : 1 No. head and 4 No. of tail.

• Combination : 2 No. head and 3 No. of tail.

• Combination : 3 No. head and 2 No. of tail.

• Combination : 4 No. head and 1 No. of tail.

• Combination : 5 No. head and 0 No. of tail.

• Repeat tossing for 100 time of a set of

five coins.

Example (cont):The probability distribution for the number of heads and tails is as follows

Example (cont.)

Finding the number of observations

Example 2: Finding no. of observations

Solution by Nomogram(Graphical) method

CONDUCTING A STUDY

• It is recommended that a uniform procedure should befollowed to perform a work sampling study .It is as follows:

1. Establish the Purpose

• First, the objective of the study should be established. Worksampling can be used to determine an overall perspective onthe work done.

2. Identify the Subjects

• Second, the people performing the task must be identified,i.e. general office work is being studied with the objective ofdetermining overall productivity.

Conducting a study(cont.)

3. Identify the Measure of Output• The 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 beingstudied. This step is especially important if the objective of the study is tomeasure productivity with the intent of setting a standard.

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

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

5. Define the Activities• This step involves defining the activities that are performed by the people

under study. For example, the definition used in a machine utilizationstudy, including only the categories of working, idle, and idle-mechanicalbreakdown.

Conducting a study(cont.)

6. Determine the Number of Observations Needed

• After the work elements are defined, the number of observations forthe desired accuracy at the desired confidence level must bedetermined. If a reasonable guess cannot be made, then a trial study ofperhaps 20 to 40 observations should be made to get an estimate.

7. Schedule the Observations

• Once the number of required observations has been determined, eitherfrom appropriate statistical calculations or from tables, and the actualobservations must be scheduled. Typically, the analyst will assign anequal number of observations each day during the course of the study.For example, if 800 observations are required and 20 work days areestablished as an appropriate observation time, 40 observations shouldbe recorded each day.

• A random number table can be used to establish the random times foreach observation.

Making random observations(Example)

• To ensure that our observations are in fact made at random, we can usea random table such as the one in table 12.

• Various types of random table exist, and these can be used in differentways. In our case let us assume that we shall carry out our observationsduring a day shift of eight hours, from 7 a.m. to 3 p.m. An eight-hour dayhas 480 minutes. These may be divided into 48 ten-minute periods.

• We can start by choosing any number at random from our table, forexample by closing our eyes and placing a pencil point somewhere onthe table. Let us assume that in this case we pick, by simple chance, thenumber 11 which is in the second block, fourth column, fourth row (table12).

• We now choose any number between 1 and 10. Assume that we choosethe number 2; we now go down the column picking out every secondreading and noting it down, as shown below (if we had chosen thenumber 3, we should pick out every third figure, and so on).

• 11 38 45 87 68 20 11 26 49 05

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

• Similarly, the second 11 will also have to be discarded since it is a number that has already been picked out. We therefore have to continue with our readings to replace the four numbers we have discarded. Using the same method, that is choosing every second number after the last one (05), we now have 14 15 47 22

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

Conducting a study(cont.)

8. Inform the Personnel Involved

• Before the study is actually performed, the personnel involvedshould be informed about the objective of the study and themethodology that will be employed.

9. Record the Raw Data

• The next is the actual recording of the raw data. Although thisrecording can be performed by anyone, it is desirable that atrained analyst be employed.

• It is also very important that the observations be made at exactlythe same location every time.

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

Example: Conducting the study• Determining the scope of the study. Before making our actual observations, it is

important that we decide on the objective of our work sampling.

• The simplest objective is that of determining whether a given machine is idle orworking. In such a case, our observations aim at detecting one of twopossibilities only:

• We can, however, extend this simple model to try to find out the cause of the stoppage of the machine:

Making the observations

• So far we have taken the first five logical steps in conducting a worksampling study.

– selecting the job to be studied and determining the objectives of thestudy;

– making a preliminary observation to determine the approximatevalues of p (idle) and q (working);

– in terms of a chosen confidence level and accuracy range, determiningn (the number of observations needed) determining the frequency ofobservations, using random tables;

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

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

Work Sampling Observation Form

Time standard using Work Sampling

• For example, 8-hour day as a drill-press operator. A work sampling study might show 15% idle or 72 minutes (480*0.15=72) and 408 minutes (480*0.85)at an average performance index of 110 percent. During the day 420 pieces are produced.

Advantages of Work Sampling

• Economical

• Many operators or activities which are difficult or uneconomical to measure by time study can readily be measured by work sampling.

• Two or more studies can be simultaneously made of several operators or machines by a single study person. Ordinarily a work study engineer can study only one operator at a time when continuous time study is made.

• It usually requires fewer man-hours to make a work sampling study than to make a continuous time study. The cost may also be about a third of the cost of a continuous time study.

• No stopwatch or other time measuring device is needed for work sampling studies.

• It usually requires less time to calculate the results of work sampling study. Mark sensing cards may be used which can be fed directly to the computing machines to obtain the results just instantaneously.

Advantages of Work Sampling(cont.)

• Flexible• 6. A work sampling study may be interrupted at any time without affecting the

results.• 7. Operators are not closely watched for long period of time. This decreases

the chance of getting erroneous results for when a worker is observed continuously for a long period, it is probable that he will not follow his usual routine exactly during that period.

• Less Erroneous• 8. Observations may be taken over a period of days or weeks. This decreases

the chance of day-to-day or week-to-week variations that may affect the results.

• Operators Like It• 9. Work sampling studies are preferred to continuous time study by the

operators being studied. Some people do not like to be observed continuously for long periods of time.

• Observers Like It• 10. Work sampling studies are less fatiguing and less tedious to make on the

part of time study engineer.

Disadvantages of Work Sampling

• Work sampling is not economical for the study of a single operator or operation or machine. Also, work-sampling study may be uneconomical for studying operators or machines located over wide areas.

• Work sampling study does not provide elemental time data.• The operator may change his work pattern when he sees the study

person. For instance, he may try to look productive and make the results of study erroneous.

• No record is usually made of the method being used by the operator. Therefore, a new study has to be made when a method change occurs in any element of operation.

• Compared to stop watch time study, the statistical approach of work sampling study is difficult to understand by workers.

Case Study 1:Salespeople

Case Study 2:Assembly line employee