Chapter 14_Work Sampling

8/2/2019 Chapter 14_Work Sampling

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Chapter 14. Work Sampling 1

Work Sampling

Introduction

The work sampling method uses statistical methods to determine theproportions of total time for various activities that constitute a job.

Applications(1) Determining machine utilization(2) Determining allowances(3) Establishing standard times

Assumption: Workers are using standard methods.

Pros and Cons

Pros Cons

Reduce tension caused byconstant observation ofworkers.

Establish an average standardtime over varying workconditions.

Permit simultaneousdevelopment of standards forvarious operations.

Decrease the accuracy ofperformance rating (difficult to doa performance rating at each shortinstant).

Require a relatively large numberof random observations for high

accuracy.

Procedure

Step 1. Determine the sample size required.Step 2. Develop a random observation schedule.Step 3. Record and plot observations.Step 4. Summarize the percentages of various activities observed,

performance ratings, and volumes of production.

Step 5. Determine machine utilizations, allowances, and standard times.


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Step 1. Determining a Sample Size

1. Estimation of Percentage

A preliminary estimate ofp is needed for an activity being studied to

determine the sample size of the study.

The probability estimate can be obtained from historical data or a pilotstudy for a short period of time (two or three days).

The ratio of observations of a given activity to the total number ofobservations is used to estimate the percentage of time for the activity.

nnp i=

where: n = total number of observations

in = number of observations for activity i

2. Estimation of Accuracy (Limit of Error)

Determine the desired accuracy (tolerance, limit of error) of the resultswith a confidence level designated.

Calculation of Maximum Limit of Error (L)

=

n

pppN

n

pqpNp

)1(,,~ if 5>np and 5)1( >= pnnq

=

+

1)1(

)1(

22n

ppzpp

n

ppzpP

n

ppzL

)1(

2

=

The maximum limit of error decreases as the size of a sample increases.

Exercise. Maximum limit of a 95% confidence error where 8.0 =p and

100=n .


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3. Determination of Sample Size (N)

Adequate sample size is needed to ensure statistical significance.

2

22/ )1(

L

ppzN

=

Exercise. 8.0 =p , %2=L , and confidence level ( 1 ) = 95%.

There is a direct trade-off between the accuracy of the study and thenumber of observations needed. Thus, we need to check if the extrasample size is worth for the extra accuracy.


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Step 2. Developing Random Observation Schedule

Random observations are needed when doing a work sampling study toreduce any bias introduced by the workers anticipation of observationtimes.

(Note) Bias Sources

(1) Workers Bias: The arrival of an observer at the work centerimmediately influences the activity of the operator.

(2) Analysts Bias: There is a natural tendency for the observer to recordwhat has just happened or what will be happening, rather than what isactually happening at the exact moment of observation.

The longer period of time the time study, the better the chance ofobserving average conditions.

The frequency of daily observations depends on:(1) The number of observations required,(2) The time available to develop the data, and(3) The number of analysts available.

Exercise. How many daily observations per observer should be conductedfor a total of 900 observations if 10 days and 3 observers are available for thestudy?

Methods available for random scheduling(1) Random number table.(2) Computer program.(3) Random reminder: pocket-sized instrument which beeps at random

times.

Devilbiss Electronics, Ltd.

Exercise. Develop a random schedule of 100 observations for five workdays.


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Step 3. Recording and Plotting Observations

1. Guidelines for Observation and Recording

Make a mark on the floor to show where the analyst should stand beforemaking an observation.

Take a visual observation, and then make a written entry after leaving thework area to minimize the workers feelings of being watched and to allowthem to perform in their accustomed manner.

(Note) A video camera can be used to save the travel time of the analystand reduce bias in the study.

If the operator or machine under study is idle, the analyst must determinethe reason for the idleness, and confirm the reason with the linesupervisor before recording the data.

2. Recording Form

Design a form to record observations. Keep unavoidable delays separatefrom personal needs (trips to restroom or drinking fountain).

A standard form does not exist because each work sampling study isunique.

(e.g.)

NoRandom

TimeMachine Drilling Set-up

Material

Handling

Unavoidable

Delay

Performance

Rating

1 8:07 3 v 125

2 8:08 4 v 110

3 8:16 2 v 110

4 9:17 3 v 105

5 10:56 1 v 110

6 11:16 1 v 105

7 11:52 5 v 110

8 13:00 3 v 105

9 13:18 2 v 11010 13:20 3 v 120

11 13:22 2 v 125

12 13:28 4 v 105

13 13:30 3 v 110

14 13:38 1 v

15 13:48 5 v 100

16 13:57 3 v 105

17 14:10 2 v 110

18 15:11 2 v 105

19 15:18 5 v 110

20 15:45 4 v 110

12 3 4 1 2090

60.0% 15.0% 20.0% 5.0% 110

Total

%


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3. Plotting Percentages

Plot p values for each day on a control chart (used in statistical qualitycontrol) using 3 limits (99.73%) as the control limits.

Exercise. Determine the control limit for 6.0 =p and 500=n .

Points beyond the control limits ofp are considered as being out of control.

(1) If the case occurred due to rare reasons (e.g., complete failure ofmachine, strike), eliminate the outliers.

(2) If the case occurred due to change of the work methods, modify thecontrol chart and recalculate the sample size requirement.

Control Chart - Drilling

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 1 2 3 4 5 6 7 8 9 10

Day

p


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Step 4. Summarizing Work Sampling Observations

(e.g.) Length of the study period = 400 hrsTotal number of observations = 900

Average performance rating = 110Production volume = 2,000 pieces.

Step 5. Analyzing Machine Utilization, Allowance, Standard Time

1. Machine Utilization

Exercise. % drilling task and 95% of confidence interval

96.12

05.0 =z

n

ppzpp

n

ppzp

)1(

)1(

22

+

2. Allowance

Determine the percentage allowance for unavoidable delays.

Exercise. % unavoidable delay and 95% of confidence interval

Machine Drilling Set-upMaterial

Handling

Unavoidable

DelayTotal

1 101 26 44 3 174

2 102 34 35 6 177

3 119 34 25 5 183

4 109 24 33 8 174

5 127 30 29 6 192Total 558 148 166 28 900

% 62.0% 16.4% 18.4% 3.1% 100.0%


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3. Standard Time

Determine the observed time (OT) of each work element for a piece:

P

pTOT ii

=

where: T = total time of the study period,

ip = % element i , and

P= total production volume during the study period

Exercise: OT

P

pTOTdrilling

drilling=

P

pTOT

upset

upset

=

P

pTOT

handlingmaterial

handlingmaterial

_

_

=

Determine the normal time (NT) of the work process for a piece:

ROTNTi

i = , where R = average rating of performance

Exercise: NT

Determine the standard time (NT) of the work process for a piece:

)1( AllowanceNTST ii +=

Exercise: ST

Chapter 14_Work Sampling

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