ch16-work sampling-2012.pdf
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Work Systems and the Methods, Measurement, and Management of Work
by Mikell P. Groover, ISBN 0-13-140650-7.
2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.
Work Sampling
Sections:
1. How Work Sampling Works
2. Statistical Basis of Work Sampling
3. Application Issues in Work
Sampling
Chapter 16
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Work Systems and the Methods, Measurement, and Management of Work
by Mikell P. Groover, ISBN 0-13-140650-7.
2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.
Work Sampling
Statistical technique for determining the proportionsof time spent by subjectsin various defined
categories of activity
Subjects = workers, machines
Categories of activity = setting up a machine,producing parts, idle, etc.
For statistical accuracy
Observations must be taken at random times
Period of the study must be representative of
the types of activities performed by the
subjects
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Work Systems and the Methods, Measurement, and Management of Work
by Mikell P. Groover, ISBN 0-13-140650-7.
2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.
Historical Notes
L. H. C. Tippettintroduced the technique of worksampling in England (1927): snap reading method
R. L. Morrow- introduced the technique in US
(1941): ratio delay study
C. L. Brisleyused the term work sampling (1952)
Other names used for work sampling:
activity sampling, occurrence sampling
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Work Systems and the Methods, Measurement, and Management of Work
by Mikell P. Groover, ISBN 0-13-140650-7.
2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.
When is Work Sampling Appropriate?
Sufficient time is available to perform the study Several weeks usually required for a work
sampling study
Multiple subjects
Work sampling suited to studies involvingmore than one subject
Long cycle times for the jobs covered by thestudy
Nonrepetitive work cycles
Jobs consist of various tasks rather than asingle repetitive task
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Work Systems and the Methods, Measurement, and Management of Work
by Mikell P. Groover, ISBN 0-13-140650-7.
2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.
Example: How Work Sampling Works
A total of 500 observations taken at randomtimes during a one-week period (40 hours) on
10 identical machines with results shown below.
Category No. of observations
(1) Being set up 75
(2) Running production 300
(3) Machine idle 125
500
How many hours per week did an average
machine sped in each category?
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Work Systems and the Methods, Measurement, and Management of Work
by Mikell P. Groover, ISBN 0-13-140650-7.
2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.
Example: Solution
Proportions of time determined as number ofobservations in each category divided by 500
Time in each category determined by multiplying
proportion by total hours (40 hr)
Category Proportion Hrs per category
(1) Being set up 75/500 = 0.15 0.15 x 40 = 6
(2) Running production 300/500 = 0.60 0.60 x 40 = 24
(3) Machine idle 125/500 = 0.25 0.25 x 40 = 10
1.00 40
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Work Systems and the Methods, Measurement, and Management of Work
by Mikell P. Groover, ISBN 0-13-140650-7.
2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.
Work Sampling Applications
Machine utilization - how much time is spent bymachines in various categories of activity
Previous example
Worker utilization - how workers spend their time
Allowances for time standards - assessment of
delay components in PFD allowance factor
Average unit time - determining the average time
on each work unit
Time standards - limited statistical accuracy
when standards set by work sampling
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Work Systems and the Methods, Measurement, and Management of Work
by Mikell P. Groover, ISBN 0-13-140650-7.
2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.
Statistical Basis of Work Sampling
Binomial distribution, in which parameterp=true proportion of time spent in a given
category of activity
There are usually multiple activity categories,
so we havep1,p2, . . ,pk, . .,pKproportions forKdifferent activity categories
The binomial distribution can be approximated
by the normal distribution, where
= np
= pnp 1
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Work Systems and the Methods, Measurement, and Management of Work
by Mikell P. Groover, ISBN 0-13-140650-7.
2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.
Alternative Parameters
The parameters and can be convertedback to proportions by dividing by the number
of observations n
p = n
np
n =
n
ppp
1
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Work Systems and the Methods, Measurement, and Management of Work
by Mikell P. Groover, ISBN 0-13-140650-7.
2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.
Estimating the Proportionp
In a sampling study, we let = the proportion ofthe total number of observations devoted to an
activity category of interest
The proportion is our estimate of the truevalue of the population proportionp
To have as a good estimator ofp:
Absence of bias -- Random observation times
Low variance -- Large number of observations
p
p
p
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Work Systems and the Methods, Measurement, and Management of Work
by Mikell P. Groover, ISBN 0-13-140650-7.
2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.
Confidence Intervals
The general statement of a confidence intervalfor relative topcan be expressed as follows
Pr = 1 -
p
22 /
p
/ z
ppz
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Work Systems and the Methods, Measurement, and Management of Work
by Mikell P. Groover, ISBN 0-13-140650-7.
2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.
Confidence Intervals
This can be rearranged to the following
Pr = 1 - p/p/ zppzp 22
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Work Systems and the Methods, Measurement, and Management of Work
by Mikell P. Groover, ISBN 0-13-140650-7.
2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.
Example: Determining the Confidence Interval
Consider the previous example. Determine a 95% CIfor the proportion of time spent in setting up the
machines.
Category Proportion Hrs per category
Being setup 75/500 = 0.15 0.15 x 40 = 6
Its standard deviation is (0.15(1-0.15)/500)1/2 = 0.01597
For 95% confidence level z/2=1.96
95% confidence interval = 0.151.96(0.01597)
= [0.1187, 0.1813]
____________________
__________
____________________
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Work Systems and the Methods, Measurement, and Management of Work
by Mikell P. Groover, ISBN 0-13-140650-7.
2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.
Number of Observations Required
How many observations are required toachieve a given confidence interval about the
estimate ofp?
We need to decide two parameters:
1.Confidence level 1 -
This allows us to find the corresponding
value of z/2
2.The half-width cof the confidence interval,
defined as the desired acceptable deviationfromp
Thus, we havepc
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Work Systems and the Methods, Measurement, and Management of Work
by Mikell P. Groover, ISBN 0-13-140650-7.
2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.
Number of Observations Required
Given z/2 and c, the number of observationsrequired to achieve the specified confidence
level is given by the following
22
2
1
cp
p
zn /
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Work Systems and the Methods, Measurement, and Management of Work
by Mikell P. Groover, ISBN 0-13-140650-7.
2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.
Example: Determining the number of
observations
Previous example. Determine how many observationswill be required to estimate the proportion of time used
to setup the 10 machines in the automatic lathe
section. The confidence interval must be within 0.03
of the true proportion, which the foreman initially
estimates to be =.20. A 95% confidence level will beused.
Solution:
z/2=1.96. c=.03
n=1.962(0.2)(0.8)/0.032=683.5
684 observations are required
p
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Work Systems and the Methods, Measurement, and Management of Work
by Mikell P. Groover, ISBN 0-13-140650-7.
2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.
Determining Average Task Times
Average task time for a given work category isdetermined by computing the total time associated
with the category and then dividing by the total
count of work units produced by that category
where Tci= average task time,
pi= proportion of observations associated
with category i,TT= total time,
Qi= total quantity associated with category i
( )i
ici Q
TTpT =
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Work Systems and the Methods, Measurement, and Management of Work
by Mikell P. Groover, ISBN 0-13-140650-7.
2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.
Example: Determining average task times
We consider the same example. A total of1672 units were completed by the 10
machines and that a total of 23 setups were
accomplished during the 5-day period.
Determine (a) the average task time per workunit during production (b) the average setup
time.
Solution:
TT=40 hr (10 machines)=400 hr(a) Tc=0.60(400)/1572=9.16 min
(b) Tsu=0.15(400)/23=2.609 hr
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___________________
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Work Systems and the Methods, Measurement, and Management of Work
by Mikell P. Groover, ISBN 0-13-140650-7.
2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.
Determining Standard Times
Similar to determining average task time,except performance rating must be factored in
First determine normal time for activity i
Then determine standard time
Tstdi= Tni(1 +Apfd)
( )( )i
iini Q
PRTTpT =
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Work Systems and the Methods, Measurement, and Management of Work
by Mikell P. Groover, ISBN 0-13-140650-7.
2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.
Defining the Activity Categories
Some guidelines:
Must be defined to be consistent with
objectives of study
Must be immediatelyrecognizable by
observer
If output measures are included, then activity
categories must correlate with those measures
If more than one output measure, then an
activity category must be defined for each
Helpful to limit the number of categories to ten
or fewer
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Work Systems and the Methods, Measurement, and Management of Work
by Mikell P. Groover, ISBN 0-13-140650-7.
2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.
Work Sampling Observation Forms
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Work Systems and the Methods, Measurement, and Management of Work
by Mikell P. Groover, ISBN 0-13-140650-7.
2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.
Scheduling Observation Times
Preparing a schedule of randomized observations Improve the statistical accuracy
Reduce bias
Two commonly used strategies:
1. Randomization of the observation times throughout theentire study period
2. Sampling stratification: Total number of observations is
divided into a specified number of time periods so that
there are an equal number of samples taken each period
Observation times within each period are randomized
Reduces the variance
More convenient for observers
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Work Systems and the Methods, Measurement, and Management of Work
by Mikell P. Groover, ISBN 0-13-140650-7.
2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.
Example: Generation of random
observation times
For the machine utilization example, generate theschedule of 10 observation times for the first day. Theshift hours are 8:00 a.m. to noon, then 1:00 p.m. to 5:00p.m.
Solution: Using a random number generator, generate aset of three digit numbers from a uniform distributionbetween 1 and 999. Conversion of numbers to clocktimes:
Numbers with first digits=8,9,1,2,3 and 4 are readdirectly as the clock hour
Numbers with first digits=0 and 6 are read as clock
hours 10 and 11, respectively Numbers with first digits=5 and 7 are discarded
Numbers with second digits 6 through 9 arediscarded
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Work Systems and the Methods, Measurement, and Management of Work
by Mikell P. Groover, ISBN 0-13-140650-7.
2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.
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Work Systems and the Methods, Measurement, and Management of Work
by Mikell P. Groover, ISBN 0-13-140650-7.
2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.
Advantages of Work Sampling
Can be used to measure activities that areimpractical to measure by direct observation
Multiple subjects can be included
Requires less time and lower cost than
continuous direct observation Training requirements less than DTS or PMTS
Less tiresome and tedious on observer thancontinuous observation
Fewer aberrations than short-run observations
Being a subjectin work sampling is lessdemanding than being watched continuouslyfor a long time
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Work Systems and the Methods, Measurement, and Management of Work
by Mikell P. Groover, ISBN 0-13-140650-7.
2007 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.
Disadvantages and Limitations
Not as accurate for setting time standards as other
work measurement techniques
Therefore, not proper to set standards for incentive
pay systems
Work sampling provides less detailed information
about work elements than DTS or PMTS Usually not practical to study a single subject
Since work sampling deals with multiple subjects,
individual differenceswill be missed
Workers may be suspicious because they do notunderstand the statistical basis of work sampling
Behaviorof subjects may be influenced by the act of
observing them
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Review Questions
State whether the following statements are true or false. Ifyou find a statement false, state why it is false and correct it.
Work sampling is particularly suitable for highly repetitive
jobs with short cycle times.
False nonrepetitive jobs - long cycle time
Bias in can be reduced by randomizing the observationtimes.
True.
Determining the average task times by using work
sampling does not require performance rating.
True.
p
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