Quality Control and Improvement Chapter 9

QualityQualityControl and ImprovementControl and Improvement

Chapter 9Chapter 9

INTRODUCTION to Operation Management 4e, Schroeder

Copyright © 2008 by The McGraw-Hill Companies, Inc., All Rights Reserved.

McGraw-Hill/Irwin

Chapter OutlineChapter OutlineDesign of Quality Control SystemsDesign of Quality Control SystemsProcess Quality ControlProcess Quality ControlAttribute ControlAttribute ControlVariables ControlVariables ControlUsing Control ChartsUsing Control ChartsContinuous ImprovementContinuous ImprovementSix SigmaSix SigmaLean and Six SigmaLean and Six SigmaQuality Control and Improvement in IndustryQuality Control and Improvement in Industry

9-2

Chapter EmphasisChapter Emphasis

Process definitionProcess definitionStatistical quality controlStatistical quality controlContinuous improvementContinuous improvement

9-3

Design of Quality Control SystemsDesign of Quality Control Systems

Break down production process into Break down production process into subprocesses and “internal customers.”subprocesses and “internal customers.”Identify “Critical points” where inspection or Identify “Critical points” where inspection or measurement should take placemeasurement should take placeFour steps in designing QC systems.Four steps in designing QC systems.

9-4

Steps in Designing QC SystemsSteps in Designing QC Systems Identify critical pointsIdentify critical points

• Incoming materials & servicesIncoming materials & services• Work in processWork in process• Finished product or serviceFinished product or service

Decide on the type of measurement Decide on the type of measurement • Variables (continuous scale, diameter, weight, tensile strength)Variables (continuous scale, diameter, weight, tensile strength)• Attribute (number of defect per unit)Attribute (number of defect per unit)

Decide on the amount of inspection to be used.Decide on the amount of inspection to be used.• Sample sizeSample size

Decide who should do the inspectionDecide who should do the inspection• Worker inspection or outside inspectionWorker inspection or outside inspection

9-5

Types Of MeasurementTypes Of Measurement

Attribute measurementAttribute measurement–Product characteristic evaluated with a discrete Product characteristic evaluated with a discrete choice:choice:

Good/bad, yes/noGood/bad, yes/no

Variables measurementVariables measurement–Product characteristic that can be measured on a Product characteristic that can be measured on a continuous scale:continuous scale:

Length, size, weight, height, time, velocityLength, size, weight, height, time, velocity

9-6

When the Inspector Finds a Defect…When the Inspector Finds a Defect…

1.1. Containment: Keep the defective items from Containment: Keep the defective items from getting to the customergetting to the customer

2.2. Correction: Find the cause of the defect and Correction: Find the cause of the defect and correct it.correct it.

3.3. Prevention: Prevent the cause from Prevention: Prevent the cause from happening again.happening again.

4.4. Continuously improve the system.Continuously improve the system.

9-7

When the Inspector Finds a Defect When the Inspector Finds a Defect e.g. Strap on backpack comes loosee.g. Strap on backpack comes loose

Containment: pull the bad backpacks from the Containment: pull the bad backpacks from the line.line.

Correction: sewing machine misaligned; fix it.Correction: sewing machine misaligned; fix it.Prevention: why was it misaligned? Find out Prevention: why was it misaligned? Find out

and change system to prevent it happening and change system to prevent it happening again.again.

Continuously monitor and improve system.Continuously monitor and improve system.

9-8

Process Quality ControlProcess Quality ControlBasic assumptions (tenets) of Process Quality Basic assumptions (tenets) of Process Quality Control:Control:– Every process has random variation in it.Every process has random variation in it.– Production processes are not usually found in a state of Production processes are not usually found in a state of

control.control.

““State of Control”; what does it mean?State of Control”; what does it mean?– Unnecessary variation is eliminated.Unnecessary variation is eliminated.– Remaining variation is because of random causes.Remaining variation is because of random causes.

9-9

“It’s our job to drive the randomness out.”--Bill James, statistician for the Boston Red Sox

Wall Street Journal, 20 June 2007, p. D7.

9-10

Process Quality ControlProcess Quality Control

Assignable (special) causesAssignable (special) causes– Can be identified and correctedCan be identified and corrected

Common causesCommon causes– Occur randomlyOccur randomly– Cannot be changed unless process is redesignedCannot be changed unless process is redesigned

9-11

Attributes & VariablesAttributes & Variables

Attributes are counts, such as the Attributes are counts, such as the number (or proportion) of defects in number (or proportion) of defects in a sample.a sample.Variables are measures (mean & Variables are measures (mean & range or standard deviation) of range or standard deviation) of critical characteristics in a sample.critical characteristics in a sample.

9-12

Statistical FormulasStatistical FormulasMean (average), Standard deviation, RangeMean (average), Standard deviation, Range

Binomial distribution ( success (1), failure(0) )Binomial distribution ( success (1), failure(0) )

LCL(lower control limit), UCL(upper control limit)LCL(lower control limit), UCL(upper control limit)

20 0

max min

( ), ,

1

n ni ii i

x x xx S R x x

n n

RDLCL 3

RDLCL 3

9-13

0 (1 ),n

iip

x p pp Sn n

3 , 3LCL UCL

Process Control Chart (Figure 9.1)Process Control Chart (Figure 9.1)

x

y

Time

Upper control limit (UCL)

Center line (CL)

Lower control limit (LCL)

Average + 3 standard deviations

Quality measurement

average

Average - 3 standard deviations

9-14

Quality Control Chart (Figure 9.2)Quality Control Chart (Figure 9.2)

Stop the process; look for assignable cause

Stop the process; look for assignable causeStop the process; look for assignable cause

Stop the process; look for assignable cause

9-15

Formulas for SPC (3 Sigma)Formulas for SPC (3 Sigma)

pp-Chart-Chart

x-Bar Chartx-Bar Chart

R-ChartR-Chart

(1 )3 p ppn

RDLCL 3

RDLCL 3

RDLCL 3 RDUCL 4

RAx 2

9-16

Issues in Using Control ChartsIssues in Using Control Charts

Sample SizeSample Size– large enough to detect defectiveslarge enough to detect defectives– variables can use smaller sample sizesvariables can use smaller sample sizes

How often to sample?How often to sample?– Depends upon costDepends upon cost

Control limits vs. product specificationsControl limits vs. product specifications– Is the process capable of producing to specs?Is the process capable of producing to specs?– Are the specifications appropriate?Are the specifications appropriate?

9-17

Continuous ImprovementContinuous Improvement

Aim of continuous improvement is to reduce Aim of continuous improvement is to reduce the variability of the product or processthe variability of the product or processTechniques for continuous improvementTechniques for continuous improvement– Pareto analysisPareto analysis– Cause-and-effect (fish-bone) diagramsCause-and-effect (fish-bone) diagrams– Process capability indiciesProcess capability indicies

9-18

Pareto AnalysisPareto AnalysisTable 9.2

Defect Items# of

DefectivesPrecent

DefectiveCumulative Percentage

Loose connections 193 46.8% 46.8%Cracked connectors 131 31.8% 78.6%Fitting burrs 47 11.4% 90.0%Improper torque 25 6.1% 96.1%O-rings missing 16 3.9% 100.0%Total 412 100.0%

Note: 40 percent of the items cause 78.6 percent of the defects

9-19

Pareto Diagram (Figure 9.3)Pareto Diagram (Figure 9.3)

0

50

100

150

200

250

Looseconnections

Crackedconnectors

Fitting burrs Improper torque O-rings missing

# of

Def

ectiv

es

0.0%

20.0%

40.0%

60.0%

80.0%

100.0%

120.0%

Perc

enta

ge

9-20

Cause-and-effect Cause-and-effect (Fish-bone, Ishikawa)(Fish-bone, Ishikawa) diagram diagram (Figure 9.4)(Figure 9.4)

Looseconnections

W orkers

M ateria lconnectors

Inspection Tools

ContentNuts

K nowledgeFatigue

Training

Hose

Size

Surface defect

SizeSm allLarge

Judgment

Measurem entM easuring

tools Errors

Inspector

Experience

Tra ining

W earAdjustm ent

Torque

Air pressure

9-21

Process Capability Index Examples (Figure 9.5)Process Capability Index Examples (Figure 9.5)fre

quen

cy

process measure process measure

9-22

Computation of Computation of CCpk pk (Figure 9.6)(Figure 9.6)fre

quen

cy

process measure process measure

9-23

min ,3 3pk

LSL USLC

Six-Sigma QualitySix-Sigma QualityPioneered by Motorola in 1988 (Juran claims credit for the Pioneered by Motorola in 1988 (Juran claims credit for the idea).idea).3.4 defects per million 3.4 defects per million

Six sigma criterion is equivalent to CSix sigma criterion is equivalent to Cpkpk = 1.5 = 1.5Sample size rules become unusableSample size rules become unusableMost process are 4 sigma, e.g. payroll, prescriptions, baggage Most process are 4 sigma, e.g. payroll, prescriptions, baggage handling, journal vouchers, restaurant bills. (.62%)handling, journal vouchers, restaurant bills. (.62%)Airline fatalities are 6.4 sigmaAirline fatalities are 6.4 sigmaIRS tax advice is less than 2 sigma ( > 31%)IRS tax advice is less than 2 sigma ( > 31%)Criticism: accepts 3.4 defects/million. Is not zero defects.Criticism: accepts 3.4 defects/million. Is not zero defects.

9-24

( 4.5 7.5 ) .00034%P x

Six Sigma QualitySix Sigma QualityProcess Improvement steps of Six Sigma Process Improvement steps of Six Sigma (DMAIC):(DMAIC):

1.1. Define (process to improve, project Define (process to improve, project specification)specification)

2.2. Measure (quality variables, goals)Measure (quality variables, goals)3.3. Analyze (find root causes and alternatives)Analyze (find root causes and alternatives)4.4. Improve (process change, check for Improve (process change, check for

improvement)improvement)5.5. Control (ensure improvement is not lost over Control (ensure improvement is not lost over

time)time)

9-25

Six Sigma QualitySix Sigma QualitySix Sigma uses a project/team approach.Six Sigma uses a project/team approach.A process is selected for improvementA process is selected for improvementA cross-functional team is formed.A cross-functional team is formed.A six sigma ‘black belt’ is chosen to head the A six sigma ‘black belt’ is chosen to head the team.team. champion, master black belt, black beltchampion, master black belt, black beltThe team uses the DMAIC method for The team uses the DMAIC method for finding root causes and improving the finding root causes and improving the process.process.

9-26

Lean and Six SigmaLean and Six Sigma

Are complementary approaches to Are complementary approaches to improvement.improvement.

– Lean seeks to eliminate waste.Lean seeks to eliminate waste.– Six sigma seeks to eliminate defects.Six sigma seeks to eliminate defects.

Six sigma organization is more formal and Six sigma organization is more formal and training intensive.training intensive.Six sigma is project focused; lean is more Six sigma is project focused; lean is more broad based.broad based.

9-27

Quality Control and Improvement Quality Control and Improvement in Industryin Industry

75% use process control charts.75% use process control charts.More use of variable (x-bar and R) charts than More use of variable (x-bar and R) charts than attribute (p) charts because of sample size attribute (p) charts because of sample size requirements.requirements.““The Seven Tools of Quality Control” (see The Seven Tools of Quality Control” (see Figure 9.7)Figure 9.7)Quality control in the service industry Quality control in the service industry (SERVQUAL)(SERVQUAL)

9-28

Quality Control and Improvement Quality Control and Improvement in Industryin Industry

The seven tools of quality controlThe seven tools of quality control– FlowchartsFlowcharts– Pareto chartsPareto charts– Cause-and-effect diagramsCause-and-effect diagrams– Run (trend) chartsRun (trend) charts– HistogramsHistograms– Control chartsControl charts– Scatter diagramsScatter diagrams

9-29

Service QualityService Quality

Service measures are perceptual or subjectiveService measures are perceptual or subjectiveSERVQUALSERVQUAL most popular measure most popular measure– Tangibles (Tangibles ( 유형유형 , , 치과치과 : : 인테리어인테리어 , , 기계기계 ))– Reliability (Reliability ( 신뢰성신뢰성 , , 예약예약 , , 자리지킴자리지킴 ))– Responsiveness (Responsiveness ( 반응반응 , , 방문시 인사방문시 인사 , , 처치 후처치 후 ))– Assurance (Assurance ( 보장보장 , , 실력실력 , , 치료성과치료성과 ))– Empathy (Empathy ( 공감공감 , , 꼼꼼함꼼꼼함 , , 덜 아픔덜 아픔 , , 자세한 자세한

설명설명 ))

SummarySummaryDesign of Quality Control SystemsDesign of Quality Control SystemsProcess Quality ControlProcess Quality ControlAttribute ControlAttribute ControlVariables ControlVariables ControlUsing Control ChartsUsing Control ChartsContinuous ImprovementContinuous ImprovementSix SigmaSix SigmaLean and Six SigmaLean and Six SigmaQuality Control and Improvement in IndustryQuality Control and Improvement in Industry

9-31

End of Chapter Nine

9-32