Quality Control

Chapter 10

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10-2

You should be able to:LO 10.1Explain the need for quality controlLO 10.2Discuss the basic issues of inspectionLO 10.3List and briefly explain the elements of the control

processLO 10.4Explain how control charts are used to monitor a

process, and the concepts that underlie their useLO 10.5Use and interpret control chartsLO 10.6Perform run tests to check for nonrandomness in

process outputLO 10.7Assess process capability

Chapter 10: Learning Objectives

10-3

What is Quality Control?Quality Control

A process that evaluates output relative to a standard and takes corrective action when output doesn’t meet standardsIf results are acceptable no further action is

requiredUnacceptable results call for correction action

Inspection alone is not is generally not sufficient to achieve a reasonable level of qualityMost organization rely upon some inspection and a

great deal of process control to achieve an acceptable level of quality.

LO 10.1

10-4

InspectionAn appraisal activity that compares goods or

services to a standardInspection issues:

1. How much to inspect and how often2. At what points in the process to inspect3. Whether to inspect in a centralized or on-site

location4. Whether to inspect attributes or variables

Inspection

LO 10.2

10-5

Sampling and corrective action are only a part of the control process

Steps required for effective control: Define: What is to be controlled? Measure: How will measurement be accomplished? Compare: There must be a standard of comparison Evaluate: Establish a definition of out of control Correct: Uncover the cause of nonrandom variability

and fix it Monitor: Verify that the problem has been eliminated

Control Process

LO 10.3

10-6

Control ChartA time ordered plot of representative sample

statistics obtained from an ongoing process (e.g. sample means), used to distinguish between random and nonrandom variability

Control limitsThe dividing lines between random and nonrandom

deviations from the mean of the distributionUpper and lower control limits define the range of

acceptable variation

Control Charts: The Voice of the Process

LO 10.4

10-7

Control Charts for VariablesVariables generate data that are measuredMean control charts

Used to monitor the central tendency of a process.“x-bar” charts

Range control chartsUsed to monitor the process dispersion

R charts

LO 10.5

10-8

Establishing Control Limits

samples ofnumber

sample ofmean

means sample of Average

where

1

k

ix

x

k

xx

i

k

i

i

iR

k

R

i

k

ii

sample of Range

ranges sample of AverageR

where

R 1

LO 10.5

10-9

Used to monitor the central tendency of a process

X-Bar Chart: Control Limits

nA

RAxLCL

RAxUCL

x

x

size, sampleon basedfactor chart control a

where

Limits Controlchart x

2

2

2

LO 10.5

10-10

Used to monitor process dispersion

Range Chart: Control Limits

nD

nD

RDLCL

RDUCL

R

R

size, sampleon basedfactor chart control a

size, sampleon basedfactor chart control a

where

Limits ControlChart R

4

3

3

4

LO 10.5

10-11

Attributes generate data that are counted.p-Chart

Control chart used to monitor the proportion of defectives in a process

c-Chart Control chart used to monitor the number of

defects per unit

Control Charts for Attributes

LO 10.5

10-12

Run TestsEven if a process appears to be in control,

the data may still not reflect a random process

Analysts often supplement control charts with a run test Run test

A test for patterns in a sequence Run

Sequence of observations with a certain characteristic

LO 10.6

10-13

Nonrandom Patterns

LO 10.6

10-14

Once a process has been determined to be stable, it is necessary to determine if the process is capable of producing output that is within an acceptable range Tolerances or specifications

Range of acceptable values established by engineering design or customer requirements

Process variabilityNatural or inherent variability in a process

Process capabilityThe inherent variability of process output (process width)

relative to the variation allowed by the design specification (specification width)

Process Capability

LO 10.7

10-15

Cp

limit ification)rance(speclower toleLTL

limit tion)(specifica ranceupper tole UTL

where6

LTL - UTL

pC

LO 10.7