2. INTRODUCTIONINTRODUCTION
L. H. C. Tippet of British Cotton industryL. H. C. Tippet of British Cotton industry
Also known as:Also known as:
• Activity samplingActivity sampling
• Ratio delay studyRatio delay study
• Random observation methodRandom observation method
• Snap reading methodSnap reading method
• Observation ratio studyObservation ratio study
3. BASIC CONCEPTSBASIC CONCEPTS
& DEFINATION& DEFINATION
Work sampling is a methodology for findingWork sampling is a methodology for finding
the percentage occurrence of a certainthe percentage occurrence of a certain
activity by statistical sampling & randomactivity by statistical sampling & random
observations.observations.
4. PROBABLITY DISTRIBUTIONPROBABLITY DISTRIBUTION
• Probability based on the binomialProbability based on the binomial
distributiondistribution (p + q)(p + q)nn
= 1= 1
• Mean =Mean = npnp, Variance =, Variance = npqnpq
• As n becomes large, the binomialAs n becomes large, the binomial
distribution approaches the normaldistribution approaches the normal
distributiondistribution
• This proportion has distribution of Mean =This proportion has distribution of Mean =
pp, Variance =, Variance = pq/npq/n
5. STEPS OF WORKSTEPS OF WORK
SAMPLINGSAMPLING
1.1. Define job activitiesDefine job activities
2.2. Determine number of observations inDetermine number of observations in
work samplework sample
nn == pp(1 -(1 - pp))
zz
ee
22
wherewhere
nn == sample size (number of samplesample size (number of sample
observations)observations)
zz == number of standard deviations from meannumber of standard deviations from mean
for desired level of confidencefor desired level of confidence
ee == degree of allowable error in sampledegree of allowable error in sample
estimateestimate
pp == proportion of time spent on a work activityproportion of time spent on a work activity
6. Example of Sample SizeExample of Sample Size
CalculationCalculation
• Formula for calculation purposes simplifiesFormula for calculation purposes simplifies
to: n = Zto: n = Z22
(1-P)(P)/(L(1-P)(P)/(L22
))
A work sampling study requires 95-percent
confidence with five-percent accuracy. How
many observations are required if the
machine down time is eight percent?
n = (1.960)2*(1-0.08)*0.08/(0.05)2
n = 225,76 observations
7. STEPS OF WORKSTEPS OF WORK
SAMPLINGSAMPLING
3.3. Determine length of sampling periodDetermine length of sampling period
4.4. Conduct work sampling study,Conduct work sampling study,
record observationsrecord observations
5.5. Periodically re-compute number ofPeriodically re-compute number of
observationsobservations
8. PROCEDUPROCEDU
RERE
• Preparing for work samplingPreparing for work sampling
• Performing work samplingPerforming work sampling
• Evaluating & presenting results ofEvaluating & presenting results of
work samplingwork sampling
9. WORK SAMPLING STUDYWORK SAMPLING STUDY
PLANSPLANS
• Determining the observations needed.Determining the observations needed.
• Determining observation frequency.Determining observation frequency.
• Design the work sampling form.Design the work sampling form.
• Observations and data recording.Observations and data recording.
• Using control charts to ensure that theUsing control charts to ensure that the
observed p values are in-control.observed p values are in-control.
• Perform estimation for p value andPerform estimation for p value and
continuous improvement.continuous improvement.
10. DETERMINATION OFDETERMINATION OF
SAMPLE SIZESAMPLE SIZE
−=
=
+=
+≤≤−
−
=
n
pq
pLCL
pCenterline
n
pq
pUCL
n
qp
zpp
n
qp
zp
E
ppz
n
3
3
ˆˆ
ˆ
ˆˆ
ˆ
2
)ˆ1(ˆ2
22
αα
14. CONDUCTING THE WORK SAMPLINGCONDUCTING THE WORK SAMPLING
STUDYSTUDY
Observ
ation
Machine
idle
Machine
working
Waiting for
repairs
Waiting for
supplies
Personal
needs
Of
workers
Cutting Boring Filling Idle
15. MAKING THE OBSERVATIONSMAKING THE OBSERVATIONS
• Selecting a job to be studied & maintainingSelecting a job to be studied & maintaining
the scope of the studythe scope of the study
• Making preliminary observations toMaking preliminary observations to
determine the approximate value of p fordetermine the approximate value of p for
the activitythe activity
• In terms of chosen confidence level,In terms of chosen confidence level,
determine the number of observationsdetermine the number of observations
needed, ‘n’needed, ‘n’
• Designing record sheet to meet theDesigning record sheet to meet the
objectives of the studyobjectives of the study
16. USES OF WORK SAMPLINGUSES OF WORK SAMPLING
• To aid in determination of time standards &To aid in determination of time standards &
delay allowances.delay allowances.
• To aid in the measurement of overallTo aid in the measurement of overall
performances.performances.
• To determine the nature & extent of cycles &To determine the nature & extent of cycles &
“Peak Load” Variations in observable activity.“Peak Load” Variations in observable activity.
• To study the utilization by supervisors &To study the utilization by supervisors &
establishing goals for supervisionestablishing goals for supervision
17. • To aid in job evaluationTo aid in job evaluation
• To assist in engineering economyTo assist in engineering economy
studiesstudies
• To aid in manpower planningTo aid in manpower planning
• For appraisal of safetyFor appraisal of safety
performance.performance.
• For appraisal of organizationalFor appraisal of organizational
efficiency.efficiency.
