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Transcript of Acceptance Sampling Types Training
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
QualityQuality Control Control
Chapter 10- Chapter 10- Acceptance Sampling Acceptance Sampling
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Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
OutlineOutline
Lot-by-lot acceptance sampling plans for attributes
Acceptance sampling plans for continuous production
Acceptance sampling plans for variables
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
Learning ObjectivesLearning Objectives
When you have completed this chapter you should be able to:
Determine the sampling plan using ANSI/ASQ Z1.4.
Know the switching rules for ANSI/ASQ Z1.4. Categorize the various sampling plan
systems in terms of lot-by-lot, continuous production, attributes or variables.
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
Learning Objectives-cont’d.Learning Objectives-cont’d.
When you have completed this chapter you should be able to:
Describe the various sampling plan systems and know their function (advantages, disadvantages, purpose etc.).
Determine the sampling plan using the Dodge Romig Tables
Construct the OC Curve for a chain sampling plan.
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
Learning Objectives-cont’d.Learning Objectives-cont’d.
When you have completed this chapter you should be able to:
Be able to use the Shainin Lot Plot Method. Determine the sampling plan ANSI/ASQ S1 Determine whether a lot is accepted or
rejected using ANSI/ASQ Z1.9
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
Lot-by-Lot Acceptance Lot-by-Lot Acceptance Sampling Plans for AttributesSampling Plans for Attributes
Devised in 1942 at Bell Labs Later became MIL-STD-105E ISO-2859 ANSI/ASQ Z1.4
Wording & terminology changesAdditional tables added
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
ANSI/ASQ Z1.4ANSI/ASQ Z1.4 This standard is applicable to:
End itemsComponents and raw materialsOperationsMaterials in processSupplies in storageMaintenance operationsData or records
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
ANSI/ASQ Z1.4ANSI/ASQ Z1.4 Intended to be used for a continuing series of lots, but may be designed for isolated lots.
Standard provides for single, double, and multiple sampling plans.
Provision is also provided for normal, tightened, or reduced inspection.
Plan is specified by the AQL, and sample-size code.
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
ANSI/ASQ Z1.4ANSI/ASQ Z1.4 Normal inspection is used at the start of
inspection with changes being a function of recent quality history.
Tightened inspection:Generally used when producer’s recent
quality history has deteriorated.
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
ANSI/ASQ Z1.4ANSI/ASQ Z1.4
Reduced inspection:Used when the producer’s recent
quality history has been exceptionally good.
Decision concerning what type of plan to use is left to the responsible authority.
Nonconformities are classified such as critical, major…
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
ANSI/ASQ Z1.4ANSI/ASQ Z1.4
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
Sample Size Code LettersSample Size Code Letters
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
Comparison of Inspection Comparison of Inspection levelslevels
III
1086
Percent Nonconforming (100Pa)
100
95
80
60
40
20
0
100=10
= 0.05
Perc
ent o
f Lot
s Ac
cept
ed (1
00P a
)
42
II I
I = 1/2 nII = nIII = 2n
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
ANSI/ASQ Z1.4ANSI/ASQ Z1.4ANSI/ASQ Z1.4ANSI/ASQ Z1.4
AQLAQLThe AQL is the most important part of the
standard since the AQL and sample size dictate the plan used.
AQL is defined as the maximum percent nonconforming that can be considered satisfactory as a process average.Satisfactory = Producer’s risk, αUsually 0.05
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
ANSI/ASQ Z1.4ANSI/ASQ Z1.4ANSI/ASQ Z1.4ANSI/ASQ Z1.4
AQLAQLAQL is specified by contract or responsible authority.
AQL may be determine from historical data, empirical judgment, engineering information, experimentation, producer’s capability, consumer’s requirements.
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
ANSI/ASQ Z1.4ANSI/ASQ Z1.4ANSI/ASQ Z1.4ANSI/ASQ Z1.4
AQLAQL AQL is specified by contract or responsible
authority.AQL may be determine from historical
data, empirical judgment, engineering information, experimentation, producer’s capability, consumer’s requirements.
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Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
ANSI/ASQ Z1.4ANSI/ASQ Z1.4
Sample SizeSample SizeDetermined by lot size and
inspection levelUse of sample-size code
Inspection level is determined by the responsible authority.
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Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
ANSI/ASQ Z1.4ANSI/ASQ Z1.4 Sample SizeSample Size
Different levels of inspection provide approximately the same protection to the producer, but different protections to the consumer.
Plan provides for “special” levels to be used where relatively small sample sizes are necessary and large sampling risks must be tolerated. (S-1, S-2, S-3, S-4) 18
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
Implementation
Determine lot sizeDetermine inspection levelFind sample-size code letter in tableDetermine AQL
ANSI/ASQ Z1.4ANSI/ASQ Z1.4
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Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
Implementation cont’d.
Determine type of sampling planFind sampling plan in appropriate
tableStart with normal inspection and
change to tightened or reduced based on switching rules
ANSI/ASQ Z1.4ANSI/ASQ Z1.4
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Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
What an AQL means is that as long as a supplier maintains his/her process average (% defective) at the assigned AQL or lower, there is a very high probability that shipments from that supplier, when inspected using ASQ Z1.4 sampling plans, will be accepted.
AQLAQL
21
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
By the same token, there is a very high probability that shipments, when inspected using ASQ Z1.4 sampling plans, will be rejected if a supplier's process average (% defective) remains higher than the assigned AQL.
AQLAQL
22
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
Single sampling plans
Start by identification of AQL, lot size, inspection level, and type of sampling plan.
Locate appropriate plan parameters in tables.
Double and Multiple Sampling
Similar procedures as single sampling
ANSI/ASQ Z1.4ANSI/ASQ Z1.4
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Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
Normal, Tightened, and Reduced Inspection Start with normal inspection
Switching Procedures Normal to tightened
Institute when 2 out of 5 consecutive lots not accepted on original inspection
Tightened to normalInstitute when 5 consecutive lots acceptedIf not, then discontinue inspection under this
plan
ANSI/ASQ Z1.4ANSI/ASQ Z1.4
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Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
Switching ProceduresNormal to reduced
Institute when: (all of the following)Preceding 10 lots on normal inspection
have been acceptedTotal nonconforming in preceding 10
samples is less than number identified in Table 10-5.
Production is at a steady rateReduced inspection is considered
desirable by responsible authority
ANSI/ASQ Z1.4ANSI/ASQ Z1.4
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Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
Switching ProceduresSwitching Procedures Reduced to normalReduced to normal
Institute when: (any of the following)Institute when: (any of the following)A lot is not acceptedA lot is not acceptedSampling procedure terminates with Sampling procedure terminates with
neither acceptance or rejection criteria neither acceptance or rejection criteria have been methave been met
Production is irregular or delayedProduction is irregular or delayedOther conditionsOther conditions
ANSI/ASQ Z1.4ANSI/ASQ Z1.4
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Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
Standard is designed for use where units of product are produced in a continuing series of lots or batches.
If a sampling plan is desirable for a lot or batch of an isolated nature, it should be chosen based on the Limiting Quality (LQ) and consumer’s risk, b. These tables are included in the standard,
but not the textbook. Or use ANSI/ASQ Standard Q3
ANSI/ASQ Z1.4ANSI/ASQ Z1.4
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Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
Standard is used for inspection of isolated lots by attributes.
Provides indexed tables by Limiting Quality, LQ.
ANSI/ASQ Standard Q3ANSI/ASQ Standard Q3
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Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
Two Schemes:
1. Used for lots that are isolated or mixed or that have an unknown history as far as all parties know.
Lot size and LQ must be known.Nominal values of the LQ are based on b=0.10
ANSI/ASQ Standard Q3ANSI/ASQ Standard Q3
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Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
Two Schemes:
2. Used when a vendor is producing a continuous stream of lots and sends one or a few to a customer who will consider them as isolated lots.
Occurs frequently when purchasing small quantities of raw materials.
ANSI/ASQ Standard Q3ANSI/ASQ Standard Q3
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Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
A set of inspection tables for lot-by-lot A set of inspection tables for lot-by-lot acceptance sampling by attributes.acceptance sampling by attributes.
Tables are based on Limiting Quality (LQ) Tables are based on Limiting Quality (LQ) and Average Outgoing Quality Limit (AOQL).and Average Outgoing Quality Limit (AOQL).
Tables for single and double samplingTables for single and double sampling Advantage that the plan provides a Advantage that the plan provides a
minimum amount of inspection for a given minimum amount of inspection for a given inspection procedure.inspection procedure.
Desirable for in-house inspectionDesirable for in-house inspection
Dodge-Romig TablesDodge-Romig Tables
31
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
Limiting Quality – LQ Based on the probability that a particular
lot, which has percent nonconforming equal to the LQ, will be accepted.Probability is the consumer’s risk, β =
0.10LQ plans give assurance that
individual lots of poor quality will rarely be accepted..
Dodge-Romig TablesDodge-Romig Tables
32
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
Observations about the LQ tables:
As the lot size increases, the relative sample size decreases.
The tables extend until the process average is one-half of the LQ.
As the process average increases, a corresponding increase occurs in the amount inspected.
Dodge-Romig TablesDodge-Romig Tables
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Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
Average Outgoing Quality Limit – AOQL
Applicable when the inspected lot is a convenient subdivision of a flow of product for materials handling purposes (non-homogeneous)
AOQL plan limits the amount of poor outgoing quality on an average basis, but gives no assurance on individual lots.
Dodge-Romig TablesDodge-Romig Tables
34
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
Process average Obtained by the same techniques as for p-
chartsFirst 25 lots the average percent
nonconforming is obtained
Type of nonconformance Dodge-Romig tables do not provide for
different categories or tightened inspectionDifferent LQ or AOQL values may be used
Dodge-Romig TablesDodge-Romig Tables
35
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
A special type of lot-by-lot acceptance sampling plan for attributes designated as “Chain Sampling Plan ChSP-1”.
Applicable to quality characteristics which involve destructive or costly tests.Use of small sample sizesAcceptance number, c=0
Poor shape of OC curve at the producer’s risk
Chain Sampling Inspection PlanChain Sampling Inspection Plan
36
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
Chain sampling uses the cumulative results of several samples
Determination of the value of i, the number of previous samples, is determined by analysis of the OC curves for a given sample size.
Chain Sampling Inspection Chain Sampling Inspection PlanPlan
37
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
Procedure:
For each lot, select a sample of size n and test each for conformance to specifications.
Chain Sampling Inspection Chain Sampling Inspection PlanPlan
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Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
Procedure: If the sample has 0 nonconforming
units, accept the lot; if the sample has 2 or more nonconforming units, do not accept the lot; and if the sample has 1 nonconforming unit, it may be accepted provided that there are 0 nonconforming units in the previous
i samples of size n.
Chain Sampling Inspection Chain Sampling Inspection PlanPlan
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n=5n=5c=0c=0 LastLast LotLot
Second-Second-to-Lastto-Last
LotLot
i previous lotsi previous lotsCurrent LotCurrent Lot
Accept if 0 Nonconforming UnitsAccept if 0 Nonconforming Unitsin in n =5n =5 or if or if 11 Nonconforming Unit Nonconforming Unit
&& Zero Nonconforming Units in Zero Nonconforming Units in iiPrevious LotsPrevious Lots
Chain Sampling DiagramChain Sampling Diagram
Figure 10-4 Chain Sampling Diagram
40
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
Conditions for use of the Chain Sampling Plan:
1.1. The lot should be one of The lot should be one of continuing series of product that continuing series of product that is sampled in substantially the is sampled in substantially the order of productionorder of production
2.2. The consumer can normally The consumer can normally expect the lots to be essentially expect the lots to be essentially the same quality.the same quality.
Chain Sampling Inspection Chain Sampling Inspection PlanPlan
41
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
Conditions for use of the Chain Sampling Plan:
3.3. The consumer has confidence in the The consumer has confidence in the producer not to occasionally send an producer not to occasionally send an unacceptable lot that would have unacceptable lot that would have the optimum chance of acceptance.the optimum chance of acceptance.
4.4. The quality characteristic is one that The quality characteristic is one that involves destructive or costly tests.involves destructive or costly tests.
Chain Sampling Inspection Chain Sampling Inspection PlanPlan
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Used for costly or destructive tests Subgroup size of 1 – item by item plan
Sequential sampling is similar to multiple sampling, except sequential sampling can, in theory, continue indefinitely. In practice, the plan is truncated after the
number inspected is equal to three times the number inspected by a corresponding single sampling plan.
Based on the Sequential Probability Ratio Test
Sequential SamplingSequential Sampling
43
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Sequential Sampling PlanSequential Sampling Plan
44
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Sequential Sampling PlanSequential Sampling Plan
45
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Designated SkSP-1 and based on AOQL.Designated SkSP-1 and based on AOQL. A single sampling plan for minimizing A single sampling plan for minimizing
inspection costs when there is a inspection costs when there is a continuing supply of lots of raw continuing supply of lots of raw materials, component parts, materials, component parts, subassemblies, and finished parts subassemblies, and finished parts from from the same sourcethe same source..
Applicable to chemical and physical Applicable to chemical and physical characteristics that require laboratory characteristics that require laboratory analysis.analysis.
Skip Lot SamplingSkip Lot Sampling
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When i consecutive lots areWhen i consecutive lots arefound to be conformingfound to be conforming
Begin by inspecting every lotBegin by inspecting every lot
Discontinue inspection of every lotDiscontinue inspection of every lotInspect a fraction, f, of the lotsInspect a fraction, f, of the lots
In a random mannerIn a random mannerWhen an inspected lot is found When an inspected lot is found
nonconformingnonconforming
Procedure for SkSP-1 PlansProcedure for SkSP-1 Plans
Figure 10-7 Procedure SkSP-1 plans 47
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
Provides procedures to reduce the inspection effort when the supplier’s quality is superior.
A skip-lot scheme used in conjunction with attribute lot-by-lot plans given in ASQ Z1.4
ANSI/ASQ S1ANSI/ASQ S1
48
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Requirements to use ANSI/ASQ S1
1. Have a documented system for controlling product quality and design changes
2. Have instituted a system that is capable of detecting and correcting changes that might adversely affect quality.
3. Not have experienced an organizational change that might adversely affect quality
ANSI/ASQ S1ANSI/ASQ S1
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Additional requirements of the product:Be of stable designHave been manufactured on a continuous
basis for at least 6 months unless agreed to a longer period.
Have been on normal and reduced inspection during the qualification period.
Have maintained a quality level at or less than the AQL for at least 6 months.
ANSI/ASQ S1ANSI/ASQ S1
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Other:The previous 10 or more consecutive lots
have been acceptedThe min cumulative sample size for the last
10 or more consecutive lots have been met.The acceptance numbers for the last 2 lots
have been met
ANSI/ASQ S1ANSI/ASQ S1
51
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Sampling for Continuous Sampling for Continuous ProductionProduction
51
Acceptance sampling for continuous production implies that manufacturing operations do not create lots as part of the normal part of the production process.
Parts are produced by a continuous process on a conveyor or similar system..
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
Plans for continuous production consist of alternating sequences of sampling inspection and screening (100%) inspection.Plans begin with 100% inspection and
convert to sampling after a stated number of items are free of nonconformities.
Sampling continues until a stated number of nonconforming units are found, at which time 100% inspection in reinstated.
53
Sampling for Continuous Sampling for Continuous ProductionProduction
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
Applicable to attribute, nondestructive inspection of moving product.
Inspection should be easy and rapid Process must be capable of
manufacturing homogeneous product
Sampling for Continuous Sampling for Continuous ProductionProduction
54
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Developed by Dodge
CSP-1, CSP-2, CSP-3Later, MIL-STD-1235B
Sampling for Continuous Sampling for Continuous ProductionProduction
55
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
CSP-1
Begins with 100% inspection of the product in the order of production until a certain number of successive units are free of nonconformities.
56
Sampling for Continuous Sampling for Continuous ProductionProduction
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
CSP-1
Sampling inspection is then instituted
Samples are selected in such a manner as to minimize any bias
If a nonconformity occurs, sampling is discontinued and 100% inspection begins..
Sampling for Continuous Sampling for Continuous ProductionProduction
57
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
The Screening Crew Inspects 100% of the Units
When i Consecutive Units Are Found Free ofNonconformities Concerned,
The Screening Crew is Released from 100% Inspectionand the Sampling Inspector Inspects a Fraction, f, ofthe Units, Where the Sample Units are Selected in aRandom Manner.
When the Sampling Inspector Finds One of theNonconformities Concerned,
Start
CSP-1 and CSP-F PlansCSP-1 and CSP-F Plans
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Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
Standard is composed of five different continuous sampling plans.
Designated by code letters
MIL-STD-1235BMIL-STD-1235B
59
Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
Inspection is by attributes for nonconforming units using three classes of severity:
CriticalMajorMinor
Each of the plans incorporates a provision for discontinuance of inspection
MIL-STD-1235BMIL-STD-1235B
60
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Shainin Lot Plot ANSI/ASQ Z1.9-2003
Acceptance Sampling Plans Acceptance Sampling Plans for Variablesfor Variables
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Besterfield: Quality Control, 8th ed.. © 2009 Pearson Education, Upper Saddle River, NJ 07458.All rights reserved
Shainin Lot Plot
Random sample of 10 subgroups of 5 each
Find average and rangeConstruct a histogramCalculate Xdouble bar, R barCalculate ULL and LLLCompare to lot plots in Figure 10-15
Acceptance Sampling Plans Acceptance Sampling Plans for Variablesfor Variables
62
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Acceptance Sampling Plans Acceptance Sampling Plans for Variablesfor Variables
63
ANSI/ASQ Z1.9-2003 Sampling Procedures and Tables for Inspection by Variables for Percent Nonconforming is an acceptance sampling system to be used on a continuing stream of lots for Acceptance Quality Limit (AQL) specified.
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Acceptance Sampling Plans Acceptance Sampling Plans for Variablesfor Variables
64
ANSI/ASQ Z1.9-2003 provides tightened, normal, and reduced plans to be used on measurements which are normally distributed. Variation may be measured by sample standard deviation, sample range, or known standard deviation. It is applicable only when the normality of the measurements is assured.