Statistical Process ControlStatistical Process Control

Operations Management - 5th EditionOperations Management - 5th Edition

Chapter 4Chapter 4

Roberta Russell & Bernard W. Taylor, IIIRoberta Russell & Bernard W. Taylor, III

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Basics of Statistical Basics of Statistical Process ControlProcess Control

Statistical Process Control Statistical Process Control (SPC)(SPC) Monitoring production process Monitoring production process

to detect and prevent poor to detect and prevent poor qualityquality

SampleSample Subset of items produced to Subset of items produced to

use for inspectionuse for inspection Control ChartsControl Charts

Process is within statistical Process is within statistical control limitscontrol limits

UCLUCL

LCLLCL

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VariabilityVariability

RandomRandom Inherent in a processInherent in a process Can be eliminated Can be eliminated

only through only through improvements in the improvements in the systemsystem

Non-RandomNon-Random Special causesSpecial causes Due to identifiable Due to identifiable

factorsfactors Can be modified Can be modified

through operator or through operator or management actionmanagement action

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SPC in TQMSPC in TQM

SPCSPC Tool for identifying problems and Tool for identifying problems and

making improvementsmaking improvements Contributes to the TQM goal of Contributes to the TQM goal of

continuous improvementscontinuous improvements

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Quality MeasuresQuality Measures

AttributeAttribute a product characteristic that can be a product characteristic that can be

evaluated with a discrete responseevaluated with a discrete response good – bad; yes - nogood – bad; yes - no

VariableVariable a product characteristic that is continuous a product characteristic that is continuous

and can be measuredand can be measured weight - lengthweight - length

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Nature of defect is different in servicesNature of defect is different in services Service defect is a failure to meet Service defect is a failure to meet

customer requirementscustomer requirements Monitor times, customer satisfactionMonitor times, customer satisfaction

Applying SPC to ServiceApplying SPC to Service

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Applying SPC to Applying SPC to Service (cont.)Service (cont.)

HospitalsHospitals Timeliness and quickness of care, staff responses to Timeliness and quickness of care, staff responses to

requests, accuracy of lab tests, cleanliness, courtesy, requests, accuracy of lab tests, cleanliness, courtesy, accuracy of paperwork, speed of admittance and checkoutsaccuracy of paperwork, speed of admittance and checkouts

Grocery StoresGrocery Stores Waiting time to check out, frequency of out-of-stock items, Waiting time to check out, frequency of out-of-stock items,

quality of food items, cleanliness, customer complaints, quality of food items, cleanliness, customer complaints, checkout register errorscheckout register errors

AirlinesAirlines Flight delays, lost luggage and luggage handling, waiting time Flight delays, lost luggage and luggage handling, waiting time

at ticket counters and check-in, agent and flight attendant at ticket counters and check-in, agent and flight attendant courtesy, accurate flight information, passenger cabin courtesy, accurate flight information, passenger cabin cleanliness and maintenancecleanliness and maintenance

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Where to Use Control ChartsWhere to Use Control Charts

Process has a tendency to go out of controlProcess has a tendency to go out of control Process is particularly harmful and costly if it goes Process is particularly harmful and costly if it goes

out of controlout of control ExamplesExamples

At the beginning of a process because it is a waste of At the beginning of a process because it is a waste of time and money to begin production process with bad time and money to begin production process with bad suppliessupplies

Before a costly or irreversible point, after which product Before a costly or irreversible point, after which product is difficult to rework or correctis difficult to rework or correct

Before and after assembly or painting operations that Before and after assembly or painting operations that might cover defectsmight cover defects

Before the outgoing final product or service is deliveredBefore the outgoing final product or service is delivered

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Control ChartsControl Charts

A graph that establishes A graph that establishes control limits of a control limits of a processprocess

Control limitsControl limits Upper and lower bands of Upper and lower bands of

a control charta control chart

Types of chartsTypes of charts AttributesAttributes

p-chartp-chart c-chartc-chart

VariablesVariables range (R-chart)range (R-chart) mean (x bar – chart)mean (x bar – chart)

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Process Control Process Control ChartChart

11 22 33 44 55 66 77 88 99 1010Sample numberSample number

UpperUppercontrolcontrol

limitlimit

ProcessProcessaverageaverage

LowerLowercontrolcontrol

limitlimit

Out of controlOut of control

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Normal DistributionNormal Distribution

=0=0 11 22 33-1-1-2-2-3-3

95%

99.74%

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A Process Is in A Process Is in Control If …Control If …

1. … no sample points outside limits

2. … most points near process average

3. … about equal number of points above and below centerline

4. … points appear randomly distributed

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Control Charts for Control Charts for AttributesAttributes

p-charts uses proportion defective in a

sample

c-charts uses number of defects in an item

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p-Chartp-Chart

UCL = p + zp

LCL = p - zp

z = number of standard deviations from process averagep = sample proportion defective; an estimate of process averagep= standard deviation of sample proportion

pp = = pp(1 - (1 - pp))

nn

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p-Chart Example p-Chart Example (p.138)(p.138)

20 samples of 100 pairs of jeans20 samples of 100 pairs of jeans

NUMBER OFNUMBER OF PROPORTIONPROPORTIONSAMPLESAMPLE DEFECTIVESDEFECTIVES DEFECTIVEDEFECTIVE

11 66 .06.06

22 00 .00.00

33 44 .04.04

:: :: ::

:: :: ::

2020 1818 .18.18

200200

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p-Chart Example (cont.)p-Chart Example (cont.)

UCL = p + z = 0.10 + 3p(1 - p)

n

0.10(1 - 0.10)

100

UCL = 0.190

LCL = 0.010

LCL = p - z = 0.10 - 3p(1 - p)

n

0.10(1 - 0.10)

100

= 200 / 20(100) = 0.10total defectives

total sample observationsp =

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0.020.02

0.040.04

0.060.06

0.080.08

0.100.10

0.120.12

0.140.14

0.160.16

0.180.18

0.200.20

Pro

po

rtio

n d

efec

tive

Pro

po

rtio

n d

efec

tive

Sample numberSample number22 44 66 88 1010 1212 1414 1616 1818 2020

UCL = 0.190

LCL = 0.010

p = 0.10

p-Chart p-Chart Example Example (cont.)(cont.)

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c-Chartc-Chart

UCL = UCL = cc + + zzcc

LCL = LCL = cc - - zzcc

where

c = number of defects per sample

cc = = cc

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c-Chart (cont. – c-Chart (cont. – p.141p.141))Number of defects in 15 sample roomsNumber of defects in 15 sample rooms

1 121 122 82 83 163 16

: :: :: :: :15 1515 15 190190

SAMPLESAMPLE

cc = = 12.67 = = 12.67190190

1515

UCLUCL = = cc + + zzcc

= 12.67 + 3 12.67= 12.67 + 3 12.67= 23.35= 23.35

LCLLCL = = cc + + zzcc

= 12.67 - 3 12.67= 12.67 - 3 12.67= 1.99= 1.99

NUMBER OF

DEFECTS

4-4-2020

33

66

99

1212

1515

1818

2121

2424

Nu

mb

er o

f d

efec

tsN

um

ber

of

def

ects

Sample numberSample number

22 44 66 88 1010 1212 1414 1616

UCL = 23.35

LCL = 1.99

c = 12.67

c-Chart c-Chart (cont.)(cont.)

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Control Chart PatternsControl Chart Patterns

UCLUCL

LCLLCL

Sample observationsSample observationsconsistently above theconsistently above thecenter linecenter line

LCLLCL

UCLUCL

Sample observationsSample observationsconsistently below theconsistently below thecenter linecenter line

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Control Chart Patterns (cont.)Control Chart Patterns (cont.)

LCLLCL

UCLUCL

Sample observationsSample observationsconsistently increasingconsistently increasing

UCLUCL

LCLLCL

Sample observationsSample observationsconsistently decreasingconsistently decreasing

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Zones for Pattern TestsZones for Pattern TestsUCL

LCL

Zone A

Zone B

Zone C

Zone C

Zone B

Zone A

Process average

3 sigma = x + A2R=

3 sigma = x - A2R=

2 sigma = x + (A2R)= 23

2 sigma = x - (A2R)= 23

1 sigma = x + (A2R)= 13

1 sigma = x - (A2R)= 13

x=

Sample number

|1

|2

|3

|4

|5

|6

|7

|8

|9

|10

|11

|12

|13

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Control Chart PatternsControl Chart Patterns

8 consecutive points on one side of the center line 8 consecutive points up or down across zones 14 points alternating up or down 2 out of 3 consecutive points in zone A but still

inside the control limits 4 out of 5 consecutive points in zone A or B

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Performing a Pattern TestPerforming a Pattern Test

11 4.984.98 BB —— BB

22 5.005.00 BB UU CC

33 4.954.95 BB DD AA

44 4.964.96 BB DD AA

55 4.994.99 BB UU CC

66 5.015.01 —— UU CC

77 5.025.02 AA UU CC

88 5.055.05 AA UU BB

99 5.085.08 AA UU AA

1010 5.035.03 AA DD BB

SAMPLESAMPLE xx ABOVE/BELOWABOVE/BELOW UP/DOWNUP/DOWN ZONEZONE

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Sample SizeSample Size

Attribute charts require larger sample sizes 50 to 100 parts in a sample

Variable charts require smaller samples 2 to 10 parts in a sample