TOPIC 5TOPIC 5

WORK SAMPLING

5.1 Elemental ratio analysis

5.1.1 Elemental breakdown

5.1.2 Ratio estimates

OutlineOutline

TOPIC 5 : WORK SAMPLING

5.1.2 Ratio estimates

5.1.3 Accuracy and confidence levels

5.1.4 Sampling and sample size

5.2 Performance sampling studies

5.3 Time standard development studies

Learning Learning OutcomeOutcome

Upon successful completion of this topic, the student will be able to:

Perform:

�� Analyze elemental ratioAnalyze elemental ratio

�� Element breakdownElement breakdown

�� The accuracy and confidence levelsThe accuracy and confidence levels

�� Calculate sample sizeCalculate sample size

�� Developing time standardDeveloping time standard

Work SamplingWork Sampling

Work sampling was developed in England Work sampling was developed in England

by by L.TippetL.Tippet in the 1930s. Labor standard in the 1930s. Labor standard

is set using output and percentage (%) of is set using output and percentage (%) of

time worker spend on tasks. Involves time worker spend on tasks. Involves time worker spend on tasks. Involves time worker spend on tasks. Involves

observing worker at random time over a observing worker at random time over a

long period.long period.

Work SamplingWork Sampling

•• WorkWork SamplingSampling isis aa techniquetechnique ofof gettinggettingideaidea aboutabout utilizationutilization ofof machinesmachines oror humanhumanbeingsbeings throughthrough aa largelarge numbernumber ofofinstantaneousinstantaneous observationsobservations takentaken atatrandomrandom timetime intervalsintervals..randomrandom timetime intervalsintervals..

•• TheThe ratioratio ofof observationsobservations ofof aa givengiven activityactivitytoto thethe totaltotal numbernumber ofof observationsobservationsapproximatesapproximates thethe percentagepercentage ofof timetime thatthatactivityactivity isis inin thatthat statestate ofof action/inactionaction/inaction

Need of Need of Work SamplingWork Sampling

NeedNeed ofof workwork samplingsampling

•• ForFor efficientefficient controlling/organizationcontrolling/organization itit isisnecessarynecessary toto havehave completecomplete picturepicture ofofproductionproduction oror nonnon productionproduction timetime ofof machinemachine&& workersworkers inin workwork areaarea..&& workersworkers inin workwork areaarea..

•• ThisThis cancan bebe donedone byby directdirect andand continuouscontinuousobservationobservation ofof shopshop floorfloor

•• However,However, itit wouldwould bebe impossibleimpossible toto dodo sosounlessunless largelarge numbernumber ofof peoplepeople areare employedemployedwhichwhich wouldwould bebe unrealisticunrealistic

Need of Work SamplingNeed of Work Sampling

Need of work sampling (Need of work sampling (Con’tCon’t))

•• ThereThere for,for, alternativealternative approachapproach isis toto havehave aaglanceglance atat thethe statestate ofof givengiven shopshop floorfloor atat aamomentmoment.. SupportSupport wewe getget 8080%% workingworking && 2020%%idleidle..idleidle..

•• IfIf thisthis actionaction isis repeatedrepeated atat differencedifference timetimeintervalsintervals duringduring aa day,day, wewe maymay saysay withwith somesomeconfidenceconfidence thatthat atat anyany timetime therethere willwill bebe 8080%%machinemachine workingworking andand 2020%% idleidle.

Basis of Work SamplingBasis of Work Sampling

•• WhenWhen samplesample sizesize isis largelarge enoughenough andandobservationobservation areare mademade random,random, therethere willwill bebequitequite highhigh probabilityprobability thatthat thesethese observationsobservationsreflectreflect realreal situationsituation withwith certaincertain errorerror

•• Thus,Thus, workwork samplingsampling isis basedbased onon probabilityprobabilitywhichwhich meansmeans “the“the extentextent toto whichwhich anan eventevent isislikelylikely toto occur”occur”..

Advantages Work Advantages Work SamplingSampling

Advantages:Advantages:

�� Less expensive than time studyLess expensive than time study

��Observers need little trainingObservers need little training

��Studies can be delayed or interrupted Studies can be delayed or interrupted with little impact on resultswith little impact on results

��Worker has little chance to affect resultsWorker has little chance to affect results

�� Less intrusiveLess intrusive

Disadvantages Work Disadvantages Work SamplingSampling

�� Disadvantages:Disadvantages:

��Does not divide work elements as Does not divide work elements as completely as time studycompletely as time study

��Can yield biased results if observer Can yield biased results if observer ��Can yield biased results if observer Can yield biased results if observer does not follow random patterndoes not follow random pattern

�� Less accurate than time study, Less accurate than time study, especially when job element times are especially when job element times are shortshort

Work Work Sampling ProcedureSampling Procedure

TheThe workwork samplingsampling procedureprocedure cancan bebesummarizedsummarized inin fivefive stepssteps::

1.1. TakeTake aa preliminarypreliminary samplesample toto obtainobtainestimatesestimates ofof parameterparameter valuesvaluesestimatesestimates ofof parameterparameter valuesvalues

2.2. ComputeCompute thethe samplesample sizesize requiredrequired

3.3. PreparePrepare aa scheduleschedule forfor randomrandomobservationsobservations atat appropriateappropriate timestimes

4.4. ObserveObserve andand recordrecord workerworker activitiesactivities

5.5. DetermineDetermine howhow workersworkers spendspend theirtheir timetime

Work Work Sampling FormulaSampling Formula

Determining the sample sizeDetermining the sample size

ToTo determinedetermine thethe numbernumber ofof observationsobservations

required,required, managementmanagement mustmust decidedecide uponupon thethe

desireddesired confidenceconfidence levellevel andand accuracyaccuracy..

Determining the sample sizeDetermining the sample size

n =n =zz22 p(1 p(1 -- p)p)

hh22

wherewhere nn == required sample sizerequired sample size

zz == standard normal deviate for desired confidence standard normal deviate for desired confidence levellevel

pp == estimated value of sample proportionestimated value of sample proportion

hh == acceptable error level in percentacceptable error level in percent

Example : tossing of coinExample : tossing of coin

• 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 : 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

CombinationNo. of

CombinationsHeads (p) Tails (q)

5 0 3

4 1 17

3 2 303 2 30

2 3 30

1 4 17

0 5 3

Total 100

Proportional distribution of ‘head’ and ‘tail’ (100 tosses of the coins)

Effect of Sample SizeEffect of Sample Size

•• IfIf wewe considerablyconsiderably increaseincrease thethe numbernumber ofoftossestosses andand inin eacheach casecase tosstoss aa largelargenumbernumber ofof coinscoins atat aa time,time, wewe cancan obtainobtain aasmoothersmoother curvecurve..

•• ThisThis curvecurve isis calledcalled thethe curvecurve ofof normalnormal•• ThisThis curvecurve isis calledcalled thethe curvecurve ofof normalnormaldistributiondistribution

•• Basically,Basically, thisthis curvecurve tellstells usus that,that, inin thethemajoritymajority ofof cases,cases, thethe tendencytendency isis forfor thethenumbernumber ofof headsheads toto equalequal thethe numbernumber ofoftailstails inin anyany oneone seriesseries ofof tossestosses

Typical case: Basis of work Typical case: Basis of work

samplingsampling•• ForFor example,example, ifif 500500 instantaneousinstantaneous

observationobservation takentaken atat randomrandom intervalsintervals overoveraa fewfew weeksweeks showshow thatthat aa lathelathe operatoroperator waswasdoingdoing productiveproductive workwork inin 365365 observationsobservationsdoingdoing productiveproductive workwork inin 365365 observationsobservationsandand inin thethe remainingremaining 135135 observationsobservations hehewaswas foundfound ‘idle’‘idle’ forfor miscellaneousmiscellaneous reasonsreasons..

•• ThenThen itit cancan bebe reliablyreliably takentaken thatthat thetheoperatoroperator remainsremains idleidle ((135135//500500)) XX 100100 ==2727%% ofof thethe timetime..

Number of samples and Number of samples and

accuracyaccuracy•• TheThe accuracyaccuracy ofof thethe resultresult dependsdepends onon thethenumbernumber ofof observationsobservations..

•• However,However, inin mostmost applicationsapplications therethere isisusuallyusually aa limitlimit beyondbeyond whichwhich greatergreater

•• However,However, inin mostmost applicationsapplications therethere isisusuallyusually aa limitlimit beyondbeyond whichwhich greatergreateraccuracyaccuracy ofof datadata isis notnot economicallyeconomicallyworthwhileworthwhile..

•• WorkWork samplingsampling cancan bebe veryvery usefuluseful forforestablishingestablishing timetime standardsstandards onon bothboth directdirectandand indirectindirect laborlabor jobsjobs..

Sampling for standard timeSampling for standard time

Step 1. Define the problem

- Describe the job for which the standardtime is to be determined.

- Unambiguously state and discriminateactivities of operator on the job that wouldactivities of operator on the job that wouldentitle him to be in “working” state.

- This would imply that when operator willbe found engaged in any activity otherthan those required would entitle him tobe in “Not Working” state

Sampling for standard timeSampling for standard time

Step 2. Design the sampling plan

- Estimate satisfactory number ofobservations to be made.

- Decide on the period of study, e.g. twodays, one week, etcdays, one week, etc

- Prepare detailed plan for taking theobservations.

- Plan will include observation schedule,exact method of observing, design ofobservation sheet, route to be followed,particular person to be observed at theobservation time, etc.

Sampling for standard timeSampling for standard time

Step 3. Contact the persons concerned andtake them in confidence regardingconduct of the study.

Step 4. Make the observations at the pre-Step 4. Make the observations at the pre-decided random times about theworking/ not working state of theoperator

- When operator is in working state,determine his performance rating.

- Record these information on theobservation sheet.

Number of observation in w/sNumber of observation in w/s

•• LargeLarge numbernumber ofof observationsobservations resultsresults ininmoremore accurateaccurate findingfinding butbut thethe samesame timetimeconsumingconsuming andand costlycostly..

•• AA costcost –– benefitbenefit thethe followingfollowing methodsmethods areare•• AA costcost –– benefitbenefit thethe followingfollowing methodsmethods areareusedused forfor estimationestimation ofof thethe numbernumber ofofobservationsobservations toto bebe mademade..

•• BasedBased onon judgmentjudgment.. TheThe studystudy personperson cancandecidedecide thethe necessarynecessary numbernumber ofofobservationsobservations basedbased onon hishis judgmentjudgment..

Work SamplingWork Sampling

Estimate employees idle 25% of the timeEstimate employees idle 25% of the time

Sample should be accurate within 3%Sample should be accurate within 3%

Wants to have 95.45% confidence in the resultsWants to have 95.45% confidence in the results

n =n =zz22 p(1 p(1 -- p)p)

n =n =hh22

wherewhere nn == required sample sizerequired sample size

zz == 2 for a 95.45% confidence level2 for a 95.45% confidence level

pp == estimate of idle proportion = 25% = .25estimate of idle proportion = 25% = .25

hh == acceptable error of 3% = .03acceptable error of 3% = .03

n = = 833 observationsn = = 833 observations(2)(2)22 (.25)(.75)(.25)(.75)

(.03)(.03)22

Work Sampling Time StudiesWork Sampling Time Studies

SalespeopleSalespeople

Travel20%

Sales in person

Figure S10.3Figure S10.3

Telephone Telephone salessales12%12%

Telephone Telephone within firmwithin firm

13%13%

Lunch and Lunch and personalpersonal10%10%

Meetings Meetings and otherand other

8%8%

Paperwork17%

20%person20%

Work Sampling Time StudiesWork Sampling Time Studies

Startup/pep talkStartup/pep talk3%3%

Breaks and lunchBreaks and lunch10%10%

AssemblyAssembly--Line Line EmployeesEmployees

Figure S10.3Figure S10.3

Productive work67%

Dead time Dead time between tasksbetween tasks

13%13%

CleanupCleanup3%3%

Unscheduled tasks Unscheduled tasks and downtimeand downtime

4%4%