9 Sampling Tech 1

8/8/2019 9 Sampling Tech 1

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Introduction to

Acceptance Sampling


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Topic Outcome:

At the end of this topic, student are able to:

Discuss the importance of acceptance

sampling (AS) in modern QC system.

List down the advantages and disadvantages

of AS.

Distinguish between AS by attributes and

variables. Evaluate the economics of inspection.


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Topic Outline:

I]Introduction

II] Some Definitions

III]Areas where Inspection & Sampling are needed

IV] Methods used in performing the task ofInspection

V] Types of Sampling.

VI] Formation of Lots.

VII] Sample Selection.

VIII] Non-Accepted Lots.

IX] Economics ofInspection.


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Introduction

Sampling, Inspection & decision making of products.Sampling?

Inspection?

Decision making disposition?

Products?


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I] InspectionI] Inspection InspectionIt is a process of measuring, examining, testing,or

otherwise comparing a unit of product with requirements.

Unit of product

It may be a single article, a pair, a set, a length, an area,

an operation, a volume, a component of an end product,

or the end product itself. The unit of product may or may

not be the same as the unit of purchase, supply,

production, or shipment.

2] Products2] Products

Some Definitions


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SamplingRandomly drawing of sample(s).

Lot dispositionLot sentencing (accept or reject)

3] Sampling3] Sampling

4] Decision making4] Decision making


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Areas where inspection &

sampling are needed: (1) Incoming, supplier inspection:

QC where raw materials are moving from supplier

(vendor) to purchaser. (components & raw materials)

Process Inspection CustomerAcceptShip

Reject

Disposition of lots

Incoming

Raw Materials


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(2) Process inspection

where work-in-progress is moving from one process toanother for further processing. (operation, materials in

process, maintenance operations, data, or records)

Process Inspection CustomerAccept Ship

Scrap Rework

Reject

Disposition of lots


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(3) Final inspection

where finished product is moving from the final processto the warehouse or customer. (end items, supplies in

storage).

Process Inspection CustomerAccept Ship

Scrap Rework

Reject

Disposition of lots


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Methods used in performing

the task of inspection

Methods used in performing

the task of inspectionA. 100% inspection (every item produced is examined)

B. Sampling inspection (based on mathematical theory

of probability) by attributes or by variables

C. Adhocsampling (not based on theory of

probability,e.g. the inspection of a fixed percentage or

spot checking).

D. No inspection


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A) 100% InspectionA) 100% Inspection

This is used to eliminate the nonconformities that have

been produced from:

A process unable to meet product specification.

A process in control but not capable (with

manufacturing difficulties).

Commonly used in new processes.

This form of inspection is very costly especially if every

batch (British) or lot (US) that needs to be screened.

Lot or batch: It means inspection lot/batch a collection of unitsof product from which a sample is to be drawn and inspected to

determine conformance with the acceptability criteria, and may

differ from a collection of units designated as a lot or batch for

other purposes (e.g. production, shipment,etc.)


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It cannot eliminate all (100%) nonconformities

presented. This could be due to

Operator fatigue

Operator training

Inspection criteriaInspection method

80-90% Human error


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One of the oldest aspect of QA (1930s)

This method is most likely to be used in one of five

situations:

When test is destructive.

When cost of 100% inspection is high in relation to

the cost of passing a nonconforming unit.

When there are many similar units to be

inspected.

When information concerning producers quality isavailable.

When automated inspection in not available.


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Advantages of Acceptance Sampling:

Economy key advantages.

Less handling damage during inspection.

Applies to destructive testing.

Upgrades inspection job from monotonous piece-by-

piece decisions to lot-by-lot decisions. Indirectly motivate improvement with rejection of

entire lots instead of mere return of nonconformities.

[Put responsibility of Quality in the hands of Operating

organization and not the inspectors]


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Disadvantages of Acceptance Sampling:

Risk of accepting bad lots and rejecting good

lots.

Increases documentation and planning time.

Less information is provided about the product.

No assurance is given to the entire lot conforms to

specifications.


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Acceptance sampling is NOT a

substitute for adequate process

monitoring and control and use ofother statistical methods to drive

variability reduction.


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Important Aspects of Acceptance Sampling (AS)

It is the purpose of AS to sentence lots, notto estimate

the lot quality.

AS plans do not provide any direct form of QC. Even if all

lots are of the same quality, sampling will accept some

lots and reject others. Process controls are used to

control and systematically improve quality, but AS is not.

The most effective use of AS is notto inspect quality into

the product, but rather as an audit tool to ensure that the

output of a process conforms to requirements.


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How many samples to be drawn? Based on good sampling plan.

A Sampling Plan indicates the number of units of

product from each lot that are to be inspected (sample

size or series of sample size) and the criteria for

determining the acceptability of the lot (acceptance and

rejection numbers).

Characteristics of a Good Sampling Plan:

Protect producer against having good lots rejected.

Protect consumer against accepting bad lots.

Encourage producer to keep process in control.

Minimize sampling cost.

Provide information on product quality.


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Lot-by-lot acceptance sampling plans.


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II] Types of Sampling PlansII] Types of Sampling Plans

Attribute Sampling Plans Variable Sampling Plans

Attributes = Quality characteristics

expressed on a go-no go basis.

Variables = measurement

information (Quality characteristics

measured on a numerical scale)

A sample is taken from a lot and

each unit is classified as GOOD orNO-GOOD.

A sample is taken and a

measurement of a specifiedparameter is made on each unit.

The number of NO-GOOD is

compared with the acceptance

number in the plan.

The measurements are then

summarized into a simple statistic

(e.g. Mean). The observed value is

compared to the allowable valuedefined in the plan.

Decision is made to accept or

reject the lot

Decision is made to accept or

reject the lot


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Comparison ofAttributes and Variables Sampling PlanComparison ofAttributes and Variables Sampling Plan

Features Attributes Variables

1Inspection

required

Each item is

classified as good or

no-good

Each item would bemeasured at the specified

parameter. Higher inspection

skill would be required.

2Type of

nonconforming

Any number of

nonconformingunder one plan

One plan,one type of

nonconforming.

3 Sample size

Large depends on

the protection

required.

Smaller sample size.

4 Distribution ofmeasurements

None Usually assumed Normal.

5Process

information

Percent

Nonconforming

Valuable information on

process average and

variation.


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Lot-by-lotAcceptance Sampling PlansLot-by-lotAcceptance Sampling Plans

Attributes Variables

MIL STD 105E

ANSI/ASQ Z1.4

(civilian counterpart)

MIL STD414

ANSI/ASQ Z1.9

(civilian counterpart)

Dodge-Romig plans


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Acceptance-Sampling PlanAcceptance-Sampling Plan

It is a statement of the sample size to be used and

the associated acceptance or rejection criteria for

sentencing individual lots.

Single, Double, and Multiple Sampling Plans.

Tighten, Normal, and Reduced Sampling Plans.

It is a set of procedures consisting of acceptance-

sampling plans in which lot sizes, sample sizes, and

acceptance or rejection criteria along with the

amount of 100% inspection and sampling are

related.

Sampling SchemeSampling Scheme


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It is a unified collection of one or more acceptance-

sampling schemes

Sampling SystemSampling System


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III] Formation of LotsIII] Formation of Lots

Lot formation can influence the effectiveness of a

sampling plan.

Lots should be:

homogeneous (same machine, operator,input

material..)

as large as possible (sample sizes do not increase

as rapidly as lot sizes, a lower inspection cost results

with larger lot size). [However, it is different in Just-in-

time procurement philosophy smaller lot size].


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Conform to good packaging principles (prevent

product damage risks and additional nonconforming

during transit).

Suitable for material handling (different handling

equipment used by producer and consumer).


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IV] Sample SelectionIV] Sample Selection Sample units selected for inspection should be

representative of the entire lot.

All sampling plans are based on the premise that each

unit in the lot has an equal likelihood of being selected (to

avoid bias) Random Sampling. How does Random Sampling perform?

1) Basic technique

Assign a number to each lot (e.g. three digit number

width, height, and depth).

Generate a series of random numbers (computer,

calculator, die, random-number table).

Select a sample.


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d

h

w


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2) Stratification technique

Stratum 2

Stratum 1

Stratum 3

Stratum 1


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V] Non-Accepted LotsV] Non-Accepted Lots

What are the actions that can be taken?

Sort by production personnel.

Rectification at consumers plant by personnel

from either producers or customers plant.

Return to producer

Non-accepted lots will receive 100% inspection.

A resubmitted lot is not normally re-inspected.


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Economics of InspectionEconomics of Inspection


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An economic evaluation of the alternatives (inspection)

requires a comparison of TOTAL COSTS under each

one.

N = number of items in lot

n = number of items in sample

p = proportion nonconforming in lot

A = damage cost incurred if a defective slips through inspection.

I = inspection cost per item.

Pa = probability that a lot will be accepted by sampling plan

Alternative Total CostNo Inspection NpA

Sampling nI + (N n)pAPa + (N n)(1 Pa)I

100% Inspection NI


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Assumption:

No inspection error

Cost of replacing a nonconforming found in

inspection is borne by the producer or is small

compared to the damage

Sample size is small compared to lot size.

Inspection cost damage cost

Break-even point (pb)

A

Ipb !


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p < pb the total cost will be lowest with sampling

inspection or no inspection.

p > pb 100% inspection

Deming kp rule


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End

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9 Sampling Tech 1