Quality

ControlBy

Nitin G Shekapure

Your definition of quality?

• High price?

• Reliable?

• Attractive?

• Durable?

• Other?

Quality - a Definition

• “Fitness for use” – Joseph Juran

• “Conformance to requirements” – Philip Crosby

• Based on various factors: reliability, durability,

performance, usefulness, aesthetics, price, etc. -

Various

• “Meets customer requirements” – Various

• “Consistently meets or exceeds customer

expectations” – William Stevenson

Quality - a Definition

• “Consistently meets or exceeds customer

requirements” – David Bentley

Emphasis on requirements

Includes understanding of expectations

Implies an agreement

• explicit or Implicit

• Concept: “Do it right the first time and every time”

The Characteristics of Quality

Durability

Serviceability

Aesthetics

Perceived Quality

Performance

Features

Reliability

Conformance

Performance

• Performance refers to a product's

primary operating characteristics.

• This dimension of quality involves measurable attributes, so

brands can usually be ranked objectively on individual aspects

of performance.

• Overall performance rankings, however, are more difficult to

develop, especially when they involve benefits that not every

consumer needs. Performance is often a source of conflict

between customers and suppliers, particularly when

deliverables are not adequately defined within specifications.

Features

• Features are additional characteristics that enhance the

appeal of the product or service to the user.

• Similar thinking can be applied to features, a second

dimensions of quality that is often a secondary aspects of

performance. Features are the "bells and whistles" of

products and services, those characteristics that supplement

their basic functioning.

• Examples include free drinks on a plane.

• The line separating primary performance characteristics from

secondary features is often difficult to draw

Reliability

• Reliability is the likelihood that a product will not fail within a specific time

period. This is a key element for users who need the product to work

without fail.

• This dimension reflects the probability of a product malfunctioning or

failing within a specified time period.

• Reliability normally becomes more important to consumers as downtime

and maintenance become more expensive.

• Reliability may be closely related to performance. For instance, a product

specification may define parameters for up-time, or acceptable failure

rates. Reliability is a major contributor to brand or company image, and is

considered a fundamental dimension of quality by most end-users.

Conformance• The dimension of conformance depicts to what extent a product’s

design and operating characteristics meet established standards.

This dimension owes the most to the traditional approaches to

quality pioneered by experts like Juran.

• All products and services involve specifications of some sort. When

products are developed, these specifications are set and a target is

set, for instance the materials used or the dimension of the

product. Not only the target but also the tolerance (the range of

permitted deviation from the target) is defined.

• In service businesses, measures of conformance normally focus on

accuracy and timeliness and include counts of processing errors,

unanticipated delays and other frequent mistakes

Durability• Durability measures the length of a product’s life. When the product

can be repaired, estimating durability is more complicated. The itemwill be used until it is no longer economical to operate it. Thishappens when the repair rate and the associated costs increasesignificantly.

• Technically, durability can be defined as the amount of use one getsfrom a product before it deteriorates. After so many hours of use,the filament of a light bulb burns up and the bulb must be replaced.Repair is impossible.

• In other cases, consumers must weigh the expected cost, in bothcost and personal inconvenience, of future repairs against theinvestment and operating expenses of a newer, more reliable model.

• Durability, then, may be defined as the amount of use one gets froma product before it breaks down and replacement is preferable tocontinued repair.

• The durability and reliability are closely linked.

Serviceability

• Serviceability involves the consumer's ease of obtaining repair service.

• The responsiveness of service personnel and the reliability of service.

• Important attributes for serviceability dimension are: service warranty,

parts warranty, parts availability, number of reasonable distance to

dealer service centers, distance to service parts center-dealer, distance

to service parts center individual, length of wait for service

appointment, schedule of preventive maintenance, employees listen to

customers, information regarding repairs, courteous service centers,

repaired correctly first time, service time relative to other dealers,

warranty claims handled without argument, average repair cost/year,

extended warranty etc.

Aesthetics• The aesthetic properties of a product contribute to a company's or

brand's identity.

• Faults or defects in a product that diminish its aesthetic properties,

even those that do not reduce or alter other dimensions of quality,

are often cause for rejection.

• Aesthetics refers to how the product looks, feels, sounds, tastes or

smells. It is clearly a matter of personal judgment and a reflection of

individual preference.

• Aesthetics also refers to the "outside" feel of the product.

• Not all people prefer "rich and full" flavor or even agree on what

that means. Companies therefore have to search for a niche. On this

dimension of quality, it is impossible to please everyone.

Perceived Quality

• Perception is not always reality. Consumers do not always have

complete information about a product's or service's attributes;

indirect measures may be their only basis for comparing brands.

• A product's durability for example, can seldom be observed directly;

it usually must be inferred from various tangible and intangible

aspects of the product. In such circumstances, images, advertising

and brand names-inferences about quality rather than the reality

itself-can be critical.

• Reputation is the primary stuff of perceived quality. Its power comes

from an unstated analogy: the quality of goods in a new product line

is similar to the quality of a company's established products.

Quality Control

Quality control, or QC for short, is a process by which entities

review the quality of all factors involved in production. This

approach places an emphasis on three aspects:

1. Elements such as controls, job management, defined and well

managed processes, performance and integrity criteria, and

identification of records

2. Competence, such as knowledge, skills, experience, and

qualifications

3. Soft elements, such as personnel, integrity, confidence,

organizational culture, motivation, team spirit, and quality

relationships.

Quality Control

Quality Control refers to the checks carried out on a product by

workers, during stages of its manufacture. These checks may

include, simply ensuring that components are positioned

correctly, or that they are the correct type and size. A final

quality check should include, testing the product to ensure it is

to the correct standard, before it is distributed to the retailer /

customer. A combination of visual checks and automated

checks (by sensors and computers) are usually carried out.

Quality Assurance

Quality Assurance is the term used by manufacturers,to describe the ‘administrative system’ put in place, toensure that quality control (checks carried out byworkers on a product / component) can be carried outeffectively.

1. Quality Assurance should ensure that staff training,administrative procedures and quality monitoringof the product at various stages of manufacture, isto the highest standard.

2. Customer surveys will be carried out to ensurecustomer satisfaction, all part of quality assurance.

How do Quality Control and Quality Assurance differ?

Quality Assurance is the system of administrativeprocedures, set up to ensure that effective qualitycontrol on the production line can be carried out. Thisincludes the training of staff, so that they know how torecognise substandard work / products.

Quality Control is, carrying out regular checks on theproduction line, as a product is manufactured.Substandard components / products are rejected,ensuring that only products of the correct standard aredistributed to the customer / retailer.

There is a tendency for individual consultants and organizations toname their own unique approaches to quality control—a few of thesehave ended up in widespread use:

TerminologyApproximate

year of first useDescription

Statistical quality control (SQC)

1930sThe application of statistical methods (specifically control chartsand acceptance sampling) to quality control.

Total quality control (TQC)

1956

Popularized by Armand V. Feigenbaum in a Harvard Business Review article and book of the same name. Stresses involvement of departments in addition to production (e.g., accounting, design, finance, human resources, marketing, purchasing, sales).

Statistical process control (SPC)

1960sThe use of control charts to monitor an individual industrial process and feed back performance to the operators responsible for that process. Inspired by control systems.

Company-wide quality control (CWQC)

1968 Japanese-style total quality control

Total Quality Management (TQM)

1985Quality movement originating in the United States Department of Defense that uses (in part) the techniques of statistical quality control to drive continuous organizational improvement.

Six Sigma (6σ) 1986Statistical quality control applied to business strategy.Originated by Motorola.

Statistical Quality Control

Statistical Quality control (SQC) involves the statistical analysis

of the inspection data. Which is based on sampling and the

principle involved in normal curve.

There are two different ways of controlling the quality of the

product:

1. Through 100 % inspection, i.e., by inspecting each and item

that is produced.

2. Through sampling techniques or the use of statistical quality

control.

What is Sampling Inspection?

Sampling inspection is conducted to evaluate the quality of alot of product, through the careful examination of a small no. ofunits of products drawn from the lot.

The purpose of sampling inspection is to

1. Interpret the specification2. Measure the product3. Compare 1 with 24. Modify the provision for tightened and reduced inspection.5. Provide for the construction of one’s own sampling plans6. Utilize experience gained during previous years to the

present version

What is Sampling Inspection?

The main purpose of sampling inspection oracceptance sampling is to distinguish between goodlots and bad, and to classify the lots of productaccording to their acceptability or non-acceptability

When an inspection has been made of all the items inthe lot, it is called 100% inspection.

When inspection is made of only a part of the lot. i.e.on a sample, it is called sampling inspection

Advantage and disadvantage of sampling

100% inspection – some undesirable aspects

1. It is quite uneconomical2. It is not possible in destructive test3. It gives false assurance about the completeness of the

job inspection4. It actually sorting and therefore, result in fatigue of

the inspectors5. It may result in accepting some defective materials

due to fatigue of the inspectors6. For the same reason it may result in rejecting some

satisfactory materials/products.7. It is impracticable when the lot size is large.

Advantages of Sampling Inspection

1. Economical through less inspection lower no. of inspection2. There is less damage during inspection3. Where inspection involves destructive tests, use of sampling

technique is ‘inevitable’4. Varying quality levels can be achieved by varying the sampling

procedure5. It causes less fatigue to the inspectors6. Exert more pressure on the supplier to improve quality.

Disadvantages of Sampling Inspection1. There are risks of accepting ‘bad’ lot and rejecting the good lot2. Some extra planning and documentation is necessary3. The sample usually provides less information about the product

than the 100% inspection.

When to use acceptance sampling?

1) When testing is destructive.2) When the cost of 100% inspection is high.3) When 100% inspection is not technologically feasible or would

require so much calendar time and/or expenses.4) When there are many items to be inspected and the

inspection error rate is high.5) When the vendor has an excellent quality history, and some

reduction in inspection from 100% is desired.6) When there are potentially serious product liability risks.

Terminology

Sampling Plan: A statement of the sampling procedure and rulefor making decision about the lot.

Sampling Inspection: Inspection in which only a portion of the lotis inspected with view on making decision about accepting andrejecting the lot with respect to certain characteristics

Lot: A collection of items which inspection will be performed.

Lot Size (N): Number of items in a lot

Sample: Group of items drawn from a lot for inspection

Sample Size (n): Number of items in the sample

Terminology

Critical Defect: A defect that would render (turn in to) an itemtotally fit for use or could result in hazardous or unsafe conditionsfor individuals using or maintaining the item such as hole at thebottom of a test tube, leakage of electric current in an electricappliance etc.

Major Defect: A defect, other than critical defect that could resultin failure or materially reduce the usability of the item of productfor its intended purposes such as missing thread of a fastener etc.

Minor Defect: Defects other than critical and major defects thatdoes not materially reduce the usability of the product for itsintended purpose.

Terminology

Defective: An item the quality of which does not meet therequirement

Acceptance Number (a): The maximum allowable number ofdefectives (or count of defects) in the samples for acceptance ofthe lot.

Rejection Number (r): The minimum of defectives (or count ofdefects) in the samples for rejection of the lot.

Acceptable Quality Level (AQL): The maximum percent defective(or the number of defects per 100 items) that for the purpose ofsampling inspection, can be considered satisfactory as a processaverage.

Terminology

Lot Tolerance Percent Defective (LTPD): The percentage or

defectives or the number of defects per 100 items in a lot that can

be tolerated in only a specific proportion of lots (which is

conventionally taken as 10 percent).

Average Outgoing Quality (AOQ): The average quality (that is,

the average percentage defectives or number of defects per 100

items) in the outgoing material when all rejected lots are

concerned and after the replacement of the defectives found.

(that is the maximum AOQ)

Sampling by attributes vs. sampling by variables

Attributes("go no-go" inspection)

• Defectives-product acceptability across range

• Defects-number of defects per unit

Variable (continuous measurement)

• Usually measured by mean and standard deviation

Commonly used Sampling Plans

Sampling scheme commonly use for attributes areusually classified into four categories

1. Single Sampling Plan

2. Double Sampling Plan

3. Multiple Sampling Plan

4. Sequential Sampling Plan

Single-Sampling Plans for Attributes

A single sampling plan is defined by sample size, n, and the

acceptance number c. Say there are N total items in a lot.

Choose n of the items at random. If at least c of the items are

unacceptable, reject the lot.

N = lot size

n = sample size

c = acceptance number

d = observed number of defectives

The acceptance or rejection of the lot is based on the results

from a single sample - thus a single-sampling plan.

Single-Sampling Plans for Attributes

(n,c)

Accept the lot

Reject the lot

Sn≦C

Sn>C

(N, d)

Total number ：N

The proportion of defects :d

Where Sn is the number of the actual defects in the sample.

Double-Sampling Plans for Attributes

(n,c1 ,r1)

Acc the lot

Reject the lot

Sn1≦c1

Sn1>c1

(n1+n2, c2)

c1<Sn1<r1

Acc the lot

Reject the lot

S(n1+n2) ≦c2

S(n1+n2) >c2

(N, d)

Double-Sampling Plans for Attributes

Multiple-Sampling Plans for Attributes

……..(n,d)

Acc the lot

Reject the lot

Sn1≦c1

Sn1>c1

(n1+n2)

c1<Sn1<r1

Acc the lot

Reject the lot

S(n1+n2)<r2

S(n1+n2)≧r2

(n1+n2+n3)

c2<S(n1+n2)<r2

Sequential-Sampling Plans for Attributes

Risk Acceptable Quality Level (AQL)

Max. acceptable percentage of defectives agreed by the producer and user.

a (Producer’s risk) The probability of rejecting a good lot.

Lot Tolerance Percent Defective (LTPD) Percentage of defectives that defines consumer’s

rejection point.

(Consumer’s risk) The probability of accepting a bad lot.

Operating Characteristic Curve

n = 99

c = 4

AQL LTPD

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1 2 3 4 5 6 7 8 9 10 11 12

Percent defective

Pro

bab

ilit

y o

f ac

cep

tan

ce

=.17

(consumer’s risk)

a = .05 (producer’s risk)

Operating Characteristic (OC) Curve

1. OC curves are graphs which show the probability ofaccepting a lot given various proportions of defectsin the lot

2. X-axis shows % of items that are defective in a lot-“lot quality”

3. Y-axis shows the probability or chance of accepting alot

4. As proportion of defects increases, the chance ofaccepting lot decreases

5. Example: 90% chance of accepting a lot with 5%defectives; 10% chance of accepting a lot with 24%defectives

Operating Characteristic (OC) Curve

Operating Characteristic (OC) Curve

Consumer’s Risk (α) is the chance of accepting a lot

that contains a greater number of defects than the

LTPD limit; Type II error

This is the probability, for a given (n,c) sampling plan,

of accepting a lot with a defect level equal to the

LTPD. The consumer suffers when this occurs, because

a lot with unacceptable quality was accepted. The

symbol β is commonly used for the Type II error and

typical values range from 0.2 to 0.01.

Consumer’s Risk

Producer’s risk (β) is the chance a lot containing an

acceptable quality level will be rejected; Type I error

This is the probability, for a given (n,c) sampling plan,

of rejecting a lot that has a defect level equal to the

AQL. The producer suffers when this occurs, because a

lot with acceptable quality was rejected. The symbol α

is commonly used for the Type I error and typical

values for range from 0.2 to 0.01.

Producer’s risk