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Transcript of Quality Control McGraw-Hill/Irwin Copyright © 2012 by The McGraw-Hill Companies, Inc. All rights...
![Page 1: Quality Control McGraw-Hill/Irwin Copyright © 2012 by The McGraw-Hill Companies, Inc. All rights reserved.](https://reader038.fdocuments.us/reader038/viewer/2022103123/56649d345503460f94a0b31b/html5/thumbnails/1.jpg)
Quality Control
McGraw-Hill/Irwin Copyright © 2012 by The McGraw-Hill Companies, Inc. All rights reserved.
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You should be able to:1. List and briefly explain the elements in the control
process2. Explain how control charts are used to monitor a
process, and the concepts that underlie their use3. Use and interpret control charts4. Perform run tests to check for nonrandomness in
process output5. Assess process capability
Instructor Slides 10-2
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Quality ControlA 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
Instructor Slides 10-3
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Instructor Slides 10-4
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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
Instructor Slides 10-5
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Instructor Slides 10-6
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Typical Inspection Points:Raw materials and purchased partsFinished productsBefore a costly operationBefore an irreversible processBefore a covering process
Instructor Slides 10-7
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Effects on cost and level of disruption are a major issue in selecting centralized vs. on-site inspection Centralized
Specialized tests that may best be completed in a lab More specialized testing equipment More favorable testing environment
On-SiteQuicker decisions are renderedAvoid introduction of extraneous factorsQuality at the source
Instructor Slides 10-8
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Quality control seeksQuality of Conformance
A product or service conforms to specifications
A tool used to help in this process:SPC
Statistical evaluation of the output of a processHelps us to decide if a process is “in control” or if
corrective action is needed
Instructor Slides 10-9
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Two basic questions: concerning variability:1. Issue of Process Control
Are the variations random? If nonrandom variation is present, the process is said to be unstable.
2. Issue of Process Capability Given a stable process, is the inherent
variability of the process within a range that conforms to performance criteria?
Instructor Slides 10-10
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VariationRandom (common cause) variation:
Natural variation in the output of a process, created by countless minor factors
Assignable (special cause) variation: A variation whose cause can be identified. A nonrandom variation
Instructor Slides 10-11
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SPC involves periodically taking samples of process output and computing sample statistics:Sample meansThe number of occurrences of some outcome
Sample statistics are used to judge the randomness of process variation
Instructor Slides 10-12
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Sampling Distribution A theoretical distribution that describes the random
variability of sample statistics The normal distribution is commonly used for this
purpose
Central Limit Theorem The distribution of sample averages tends to be normal
regardless of the shape of the process distribution
Instructor Slides 10-13
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Instructor Slides 10-14
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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
Instructor Slides 10-15
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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 distribution
Upper and lower control limits define the range of acceptable variation
Instructor Slides 10-16
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Each point on the control chart represents a sample of n observations
Instructor Slides 10-17
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Type I error Concluding a process is not in control when it actually
is.The probability of rejecting the null hypothesis when the
null hypothesis is true.Manufacturer’s Risk
Type II error Concluding a process is in control when it is not.
The probability of failing to reject the null hypothesis when the null hypothesis is false.
Consumer’s Risk
Instructor Slides 10-18
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Instructor Slides 10-19
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Instructor Slides 10-20
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Variables generate data that are measured
Mean control chartsUsed to monitor the central tendency of a process.
“x- bar” charts
Range control chartsUsed to monitor the process dispersion
R charts
Instructor Slides 10-21
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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
Instructor Slides 10-22
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Used to monitor the central tendency of a process
nA
RAxLCL
RAxUCL
x
x
size, sampleon basedfactor chart control a
where
Limits Controlchart x
2
2
2
Instructor Slides 10-23
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Used to monitor process dispersion
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
Instructor Slides 10-24
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Instructor Slides 10-25
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To determine initial control limits: Obtain 20 to 25 samples Compute appropriate sample statistics Establish preliminary control limits Determine if any points fall outside of the control limits
If you find no out-of-control signals, assume the process is in control
If you find an out-of-control signal, search for and correct the assignable cause of variation
Resume the process and collect another set of observations on which to base control limits
Plot the data on the control chart and check for out-of-control signals
Instructor Slides 10-26
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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
Instructor Slides 10-27
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When observations can be placed into two categories.Good or badPass or failOperate or don’t operate
When the data consists of multiple samples of several observations each
Instructor Slides 10-28
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)ˆ(LCL
)ˆ(UCL
)1(ˆ
nsobservatio ofnumber Total
defectives ofnumber Total
pp
pp
p
zp
zpn
pp
p
Instructor Slides 10-29
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Use only when the number of occurrences per unit of measure can be counted; non-occurrences cannot be counted. Scratches, chips, dents, or errors per item Cracks or faults per unit of distance Breaks or Tears per unit of area Bacteria or pollutants per unit of volume Calls, complaints, failures per unit of time
czc
czc
c
c
LCL
UCL
Instructor Slides 10-30
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At what points in the process to use control charts
What size samples to take
What type of control chart to use
Variables
Attributes
Instructor Slides 10-31
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Even 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
Instructor Slides 10-32
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Instructor Slides 10-33
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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)
Instructor Slides 10-34
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LowerSpecification
UpperSpecification
Process variability (width) is less than the specification width
LowerSpecification
UpperSpecification
Process variability (width) matches specifications width
LowerSpecification
UpperSpecification
Process variability (width) exceeds specifications
Instructor Slides 10-35
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limit ification)rance(speclower toleLTL
limit tion)(specifica ranceupper tole UTL
where6
LTL - UTL
pC
Instructor Slides 10-36
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Used when a process is not centered at its target, or nominal, value
3
LTL,
3
UTLmin
,min
xx
CCC plpupk
Instructor Slides 10-37
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SimplifyStandardizeMistake-proofUpgrade equipmentAutomate
Instructor Slides 10-38
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Instructor Slides 10-39
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Quality is a primary consideration for nearly all customersAchieving and maintaining quality standards is
of strategic importance to all business organizationsProduct and service designIncrease capability in order to move from extensive
use of control charts and inspection to achieve desired quality outcomes
Instructor Slides 10-40