ISQA 572/ 449Models for Quality Control/

Process Control and Improvement

ISQA 572/ 449Models for Quality Control/

Process Control and Improvement

Dr. David Raffo

Tel: 725-8508, Fax: 725-5850

Email: davidr@sba.pdx.edu

AgendaAgenda

• Announcements• Questions• HW1• Control Charts (Variables) Cont.• Quality Costs• SPC Vs SQC (Inspections and

Acceptance Sampling)• Control Charts (Attributes)• Process Capability

Quality CostsQuality Costs

Quality CostsQuality Costs

Quality Cost: Traditional View TM2-5Quality Cost: Traditional View TM2-5

Competitive Benefits of TQM Exhibit 2-8Competitive Benefits of TQM Exhibit 2-8

SPC Vs SQC (Inspections and Acceptance Sampling)

SPC Vs SQC (Inspections and Acceptance Sampling)

Approaches to Quality TM 4-1Approaches to Quality TM 4-1

Quality Control Modes TM 4-2Quality Control Modes TM 4-2

Statistical Process Control: Prevention TM 4-3 Statistical Process Control: Prevention TM 4-3

Disadvantages of InspectionDisadvantages of Inspection

• Wasteful–Sampling and inspection add cost and

decrease value

• Inaccurate–Even 100% inspection is only 80%

effective because of the possibility of human errors

• Impractical(Costly)–Inspection may involve destructive

testing

Disadvantages of InspectionDisadvantages of Inspection

• Wrong message– Inspection communicates to people and

suppliers that bad parts will still be tolerated.

• Risks– In sampling and inspection there is a risk of

accepting bad lots and rejecting good lots

• No continuous improvement–Sampling is still inspection, not prevention,

so that quality is not typically continuously improved.

Advantages of a Stable ProcessAdvantages of a Stable Process

• Management and workers know the process capability and can predict performance, costs and quality levels.

• Productivity will be at a maximum and costs will be minimized.

• Management will be able to measure the effects of changes in the system with greater speed and reliability.

Advantages of a Stable ProcessAdvantages of a Stable Process

• If management wants to alter specification limits, it will have the data to back up its decision.

• (A stable process does not necessarily meet specs nor exhibit minimal variation - it’s just predictable)

Acceptance SamplingAcceptance Sampling

• Acceptance sampling has three basic decisions: accept, reject, or resample.

• Reason for using acceptance sampling:– Cost of passing defects is low

– Destructive testing is required

– Cost of inspection high relative to cost of loss

– Assumes stable process

– Large number of items must be processed in a short time

Acceptance SamplingAcceptance Sampling

• Terms– Producer’s Risk (): Risk of rejecting a lot with

acceptable quality level. (type I error)

– Consumer’s Risk (): Risk of accepting a lot with unacceptable quality level. (type II error)

– Acceptable Quality Level (AQL): The maximum percentage defective that can be considered satisfactory.

– Lot Tolerance Percent Defective (LTPD): The percent defective where the consumer desires the probability of acceptance to be at a low level.

Acceptance Sampling - AttributesAcceptance Sampling - Attributes

• Types of plans– Single

› N, n, c

› (1000, 50, 1)

– Double

› N, n1, n2, c1, c2 , c3

› (3000, 50, 80, 1, 3, 5)

– Sequential

› n, ca, cr

› (50, 0, 4); (50, 1, 5)

Acceptance Sampling - AttributesAcceptance Sampling - Attributes

• Measures– Average Outgoing Quality (AOQ)

– Average Total Inspection

– Average Sample Number

• Standard Sampling Plans– MIL-STD-105E

– Dodge-Romig

– Chain Sampling

– Skip-Lot

– Deming kp

Acceptance Sampling - VariablesAcceptance Sampling - Variables

• Advantages– Smaller sample than equivalent attribute plan

– Provides more information

– Provides insight into quality improvements

• Disadvantages– Separate plan for each variable

– Inspection costs are higher

– Distribution estimate required

Acceptance Sampling - VariablesAcceptance Sampling - Variables

• Process Parameter– Average quality of the product/process or

variability of the quality is known

– Single Specification

› n & Xa

– Double Specification

› n, XLa, XUa

• Lot Proportion Nonconforming– Form 1 (k-method)

– Form 2 (M-method)

Control Charts (Attributes)Control Charts (Attributes)

Advantages & Disadvantages of Attribute Charts

Advantages & Disadvantages of Attribute Charts

• Advantages–Some quality characteristics can only be

viewed as a attribute.

–Quality characteristic may be measurable as a variable but an attribute is used for time, cost or convenience.

–Combination of variables can be measured as an attribute rather than use a multivariate chart.

Advantages & Disadvantages of Attribute Charts

Advantages & Disadvantages of Attribute Charts

• Disadvantages–Attributes don’t measure the degree to

which specifications are met or not met.

–Doesn’t provide much information on cause.

–Variable chart can indicate potential changes which allow preventive actions.

–Larger sample size required.

Types of Attribute ChartsTypes of Attribute Charts

• p-Chart - Fraction Nonconforming–Can have constant or variable sample

size.

–Good tool for relating information about average quality level.

• np-Chart - Number of Nonconforming–Number of nonconforming items may be

easier for user to understand.

Types of Attribute ChartsTypes of Attribute Charts

• c-Chart - Number of Nonconformities–Used when desire is to control the

number of defects where one defect may not cause the entire product to be defective.

–Often used where area of opportunity is continuous and a constant size

Types of Attribute ChartsTypes of Attribute Charts

• u-Chart - Number of Nonconformities/unit–Area of opportunity is of variable size.

• U-Chart - Number of Demerits/unit–Allows the use of variable weights for

different classes of defects.

p Chart TM 4-12p Chart TM 4-12

p Chart TM 4-13p Chart TM 4-13

p Chart TM 4-14p Chart TM 4-14

p-Chart Exhibit 4-26p-Chart Exhibit 4-26

Hotel Suite Inspection - Defects Discovered Exhibit 4-27Hotel Suite Inspection - Defects Discovered Exhibit 4-27

C-Chart CalculationsC-Chart Calculations

• Centerline c-bar = ( c)/m #sub-groups• UCLc = c-bar + 3*sqrt(c-bar)• LCLc = c-bar - 3*sqrt(c-bar)

c Chart for Hotel Suite Inspection Exhibit 4-28c Chart for Hotel Suite Inspection Exhibit 4-28

Process CapabilityProcess Capability

Process Capability AnalysisProcess Capability Analysis

• Creates uniformity of output

• Level of quality is maintained or improved

• Facilitates product and process design

• Assists in supplier selection and control

• Reduces total costs

Process Capability : Normal Curve TM 4-15Process Capability : Normal Curve TM 4-15

Process Capability TM 4-17Process Capability TM 4-17

Capability IndexesCapability Indexes

• Cp

– Ability to meet two-sided specification limits

– Cp = (USL-LSL)/(6 )

– Assumes

› Stable process

› Normal distribution

› Variables data

› Centered process

– Goal Cp>1.0

Capability IndexesCapability Indexes

• Capability Ratio– CR = (6)/(USL-LSL)

– Poor if CR>1

Capability IndexesCapability Indexes

• CPU & CPL– Ability to meet one-sided specification limit

– CPU = (USL-X)/(3)

– CPL = (X-LSL)/(3)

– Assumes

› Stable process

› Normal distribution

› Variables data

Process Capability Chart Exhibit 4-20Process Capability Chart Exhibit 4-20

Capability IndexesCapability Indexes

• Cpk

– Ability to meet two sided specification but the process does not have to be centered

– Cpk = Cp - [|m-X|/(3)] where m=nominal centerline

Process Capability Index TM 4-18Process Capability Index TM 4-18

Process Capability: Varieties TM 4-19Process Capability: Varieties TM 4-19C pk

Questions?