1 ies 331 quality control chapter 14 acceptance sampling for attributes – single sampling plan and...

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IES 331 Quality Control

Chapter 14Acceptance Sampling for Attributes –

Single Sampling Plan and Military Standard

Week 13 August 30 – September 1, 2005

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Dr. Karndee Prichanont IES331 1/2005

Acceptance Sampling Another area of quality control and improvement

Closely connected with inspection and testing of product

Inspection can occur at many points in a process

Acceptance Sampling: the inspection and classification of a sample of nits selected at random from a larger batch or lot and ultimate decision about disposition of the lot –

Lot Disposition or Lot Sentencing

Acceptance Sampling: the inspection and classification of a sample of nits selected at random from a larger batch or lot and ultimate decision about disposition of the lot –

Lot Disposition or Lot Sentencing

Two common points of inspection

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Acceptance Sampling

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3 approaches for sentencing

1. _________________1. _________________

2. ____________________2. ____________________

3. ___________________3. ___________________

• Inspect every item in the lot, then remove the defective units by discarding, reworking, or returning to suppliers• For critical process. Defective input may result high failure cost• From new supplier

• when 100% inspection is too costly• Trusted supplier with potential risk

• For low cost or low impact material on the subsequent process• From trusted supplier

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Advantages and Disadvantages of Sampling

Advantages

Less expensive because of less inspection

___________________

___________________

Rejection on entire lot motivates quality improvement for suppliers

Disadvantages

Risk of _______________

Risk of _______________

Requires planning and documentation

Require extensive study on customer’s requirement

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Types of Sampling Plan

Sampling Plan for VariablesSampling Plan for VariablesSampling Plan for AttributesSampling Plan for Attributes

1. Single-sampling plan

2. Double-sampling plan

3. Multiple-sampling plan

4. Sequential sampling plan

___________________________

_____________________________

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Lot Formation1. Lots should be ______________

produced on the same machines, by same operators, from common raw materials, at approximately the same time period

2. Larger lots are better than smaller lots

3. Lots should be conformable to the material handling systems ___________________________________

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Random Sampling Inspected units should be

selected at random

Inspected units should represent all items in the lot

Potential bad story

____________________

__________________

__________________

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Single-Sampling Plans for Attributes A lot of size N has been submitted for inspection

Sample size n

Acceptance number of defective c

Lot sentencing is based on one sample of size n

N = 10,000n = 89

ExampleN = 10,000n= 89c = 2

ExampleN = 10,000n= 89c = 2

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Single-Sampling Plans for Attributes

The OC Curve Operating characteristic (OC) curve

Also refer to Chapter 2; Page 78

Curve plots the probability of accepting

the lot (Pa) versus the lot fraction

defective (p)

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The OC Curve

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Ideal OC curve Theoretically , it can be

achieved by ___________________

Due to sampling, ideal OC curve cannot be perfectly achieved.

If sample size (n) _______, the OC curve shape will approach the ideal OC curve

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Effects of nn on OC curves

As sample size (n)

__________,__________, the OC curve

shape will approach the

ideal OC curve

Note that c is kept in

proportional to n

____________________

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Effects of cc on OC curves

Changing acceptance

number (c) does not

significantly change the

shape

___________________________

_______________

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OC curveAcceptable quality level (AQL)

The poorest quality level for the supplier’s process that a consumer would consider to be acceptable

A property of the supplier’s manufacturing process, not a property of the sampling plan

Lot tolerance percent defective (LTPD)

The protection obtained for individual lots of poor quality

Also called rejectable quality level (RQL) and the limiting quality level (LQL)

LTPD is a level of lot quality specified _____________, not a characteristic of the sampling plan

Sampling plans can be designed to have specified performance at the AQL and the LTPD points

______________________________________

______________________________________

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Type-A and Type-B OC Curves

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Designing a single-sampling plan with a specified OC curve

To construct sampling plan such that

Probability of acceptance for lots with fraction defective p1

Probability of acceptance for lots with fraction defective p2

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Binomial Nomograph

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Rectifying Inspection When the lot is rejected, it requires corrective action

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Average outgoing quality - AOQ

Given the lot size of N, sample size of n, and fraction defective of p

n items in the sample that, after inspection, contain no defectives, because all discovered defectives are replaced

N – n items that, if the lot is rejected, also contain no defectives

N – n items that, if the lot is accepted p(N-n) defectives

If N >> n, then

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Average outgoing quality - AOQ

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Military Standard 105E Sampling procedure for inspection by attributes developed during

World War II and is the most widely used acceptance-sampling system for attributes in the world today

A collection of sampling schemes; therefore an acceptance-sampling system

Provides for three types of sampling: single, double, and multiple

Primary focal point is the acceptable quality level (AQL)

Different AQLs may be designated for different types of defects: critical, major, and minor

Generally specified in contract or by authority responsible for sampling

Sample size is determined by lot size and by choice of inspection level

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Military Standard 105E

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Military Standard 105E Procedure1. Choose the AQL

2. Choose the inspection level

3. Determine the lot size

4. Fine the appropriate sample size code letter (from Table 14-4)

5. Determine the appropriate type of sampling plan to use (single)

6. Enter the appropriate table to find the type of plan to be used

7. Determine the corresponding normal and reduced inspection plans to be used when required

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Discussion of Military Standard Several points about MIL STD 105E should be emphasized:

MIL STD 105E is AQL-oriented

Not all possible sample sizes are possible (2,3,5,8,13,20,32,50, etc.)

Sample sizes are related to lot sizes

Switching rules are subject to criticism for both misswitching between inspection plans and discontinuation even though there has been no actual quality deterioration

1 ies 331 quality control chapter 14 acceptance sampling for attributes – single sampling plan and...

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acceptance sampling

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lot of size n

process acceptance sampling

variables sampling plan

doublesampling plan

multiplesampling plan