Study Notes: Quality Models

Table of Contents

  1. Part 1: What is Quality? (Garvin Framework)
  2. Part 2: Quality Models & Terminology
  3. Part 3: Classic Quality Models
  4. Part 4: Modern Practice (ISO + Requirements)
  5. Part 5: Quality Gurus
  6. Part 6: Summary & Review

Part 1: What is Quality? (Garvin Framework)

Learning Objectives

  • Understand that quality is multi-dimensional
  • Recognize that different stakeholders define quality differently
  • Apply Garvin’s five views to real-world products

Garvin’s Five Views of Quality

David Garvin [1] identified five distinct perspectives on quality (see Views for full details). Each leads to different quality management approaches.

View Definition Example Approach
Transcendent “I know it when I see it” Rolex watch, Monet painting Subjective judgment
Product-based More of attribute X is better Smartphone specs, diamond carats Measurable attributes
User-based Fitness for purpose Toyota Corolla (reliable, low-maintenance) Customer satisfaction
Manufacturing Conformance to specifications Zero defects in assembly Process control
Value-based Quality at acceptable cost Budget airline ticket ROI analysis

Detailed View Descriptions

1. Transcendent View

  • Quality is absolute and universally recognizable but cannot be precisely defined
  • Often associated with excellence or “innate goodness”
  • Example: A luxury watch like a Rolex is recognized as high quality because of its superior craftsmanship and prestige
  • Criticism: Subjective and difficult to measure or operationalize

2. Product-Based View

  • Quality is a precise and measurable variable based on product attributes
  • More features or better performance equals higher quality
  • Example: A smartphone with better resolution, longer battery life, and faster processing speed
  • Criticism: Assumes all improvements are desirable, ignoring trade-offs like cost

3. User-Based View

  • Quality is determined by the extent to which a product satisfies customer needs
  • Different users = different quality definitions
  • Example: A budget-friendly car like Toyota Corolla is high quality to customers who value reliability
  • Criticism: Highly subjective and dependent on individual expectations

4. Manufacturing-Based View

  • Quality is defined as conformance to specifications or being free from defects
  • Focus on process excellence
  • Example: A car produced with zero defects in the assembly line
  • Criticism: Focuses on internal consistency but overlooks customer satisfaction

5. Value-Based View

  • Quality is the balance between performance and cost
  • Delivers the most value for its price
  • Example: A budget airline ticket that meets expectations for basic transportation at low price
  • Criticism: Overemphasis on cost-performance trade-offs can compromise safety

Practical Three-View Model

Most software organizations blend three views:

Perspective Definition Methodology Emphasis
User-based “Fitness for purpose” (Juran) Agile
Manufacturing “Conformance to requirements” (Crosby) CMMI, process improvement
Value-based Quality at acceptable cost Business perspective

Part 2: Quality Models & Terminology

Learning Objectives

  • Define quality model and explain why it’s needed
  • Understand the hierarchy: Characteristics → Attributes → Measures
  • Distinguish between QA, QC, and related terms

Why Quality Models Are Needed

Quality models solve the “I know it when I see it” problem:

  1. Shared vocabulary — Common language for quality requirements
  2. Measurement capability — Enable comparison and tracking
  3. Design guidance — Inform development decisions
  4. Trade-off analysis — Framework for understanding conflicts

Quality Model Definitions

Simple Definition:

A model with the objective to describe, assess, and/or predict quality.

ISO 25000 Definition:

A defined set of characteristics, and of relationships between them, which provides a framework for specifying quality requirements and evaluating quality.

Full Definition:

A structured specification of quality in terms of characteristics, attributes, their relationships, and corresponding measurement scales and methods.

Quality Model Hierarchy 

Quality Model
├── Quality Characteristic (e.g., Reliability)
│   ├── Quality Attribute (e.g., Fault Tolerance)
│   │   ├── Measurement Scale
│   │   └── Measurement Method
│   └── Quality Attribute (e.g., Recoverability)
└── Quality Characteristic (e.g., Usability)
    └── ...

Key Distinction:

  • Characteristics are high-level (not directly measurable)
  • Attributes can be measured

Internal vs External Quality

Type Visibility Who Sees It Examples
Internal (white-box) Developers Code metrics, cyclomatic complexity, code coverage  
External (black-box) Users Response time, availability, usability  

Key Terminology

Term Definition Key Point
Quality Management (QM) Plan, verify, remediate, improve Overall umbrella term
Quality Planning (QP) Identify requirements, activities, tools Front-end planning
Quality Assurance (QA) Provide confidence in quality Focuses on process
Quality Control (QC) Verify and correct Focuses on product
Quality Improvement (QI) Enhance process effectiveness Continuous improvement

Part 3: Classic Quality Models

Learning Objectives

  • Explain McCall’s quality model structure (factors, criteria, metrics)
  • List and categorize McCall’s 11 quality factors
  • Understand quality attribute tradeoffs (Perry’s model)

McCall’s Quality Model

McCall [2] developed the first comprehensive software quality model for the US Air Force (see Models for full details).

Innovation: First structured approach to software quality Structure: 11 quality factors in 3 categories

The Three Categories

Category Focus Question What It Covers
Product Operation How well does it run? Day-to-day functionality
Product Revision Can we change it? Maintenance and evolution
Product Transition Can we move it? Portability and integration

McCall’s 11 Quality Factors

Category Factors Description
Product Operation Correctness Meets specified requirements
  Reliability Performs without failure
  Efficiency Uses minimal resources
  Integrity Protected from unauthorized access
  Usability Easy to learn and operate
Product Revision Maintainability Easy to modify for corrections
  Flexibility Easy to modify for enhancements
  Testability Easy to validate and verify
Product Transition Portability Runs in different environments
  Reusability Components can be reused
  Interoperability Works with other systems

Three-Level Structure

McCall’s model has three levels:

  1. Factor — External view (what users see)
    • Example: Reliability
  2. Criteria — Internal view (what developers control)
    • Example: Simplicity, Modularity
  3. Metric — Measurement
    • Example: Cyclomatic complexity

Key Insight: Same criterion can affect multiple factors!

  • Example: Modularity → Maintainability AND Testability

Perry’s Quality Attribute Relationships

Perry [3] showed that quality attributes are not independent — they have relationships:

Relationship Type Symbol Meaning
Direct + Both improve together
Inverse − Tradeoff (one improves, other worsens)
Neutral (blank) No relationship

Key Tradeoffs to Know

Attribute Pair Relationship Explanation
Integrity vs Efficiency Inverse (−) Access control adds overhead
Usability vs Efficiency Inverse (−) Better UI requires more code
Flexibility vs Integrity Inverse (−) Flexible data = security risks
Portability vs Reusability Direct (+) Both benefit from good structure
Maintainability vs Testability Direct (+) Well-structured code is easier to test

Perry’s Tradeoff Matrix (Selected)

Attribute Efficiency Maintainability Reusability
Integrity −   −
Usability − +  
Testability − + +
Portability − + +

Key Takeaway: You cannot maximize all quality attributes simultaneously. Quality engineering is about informed tradeoffs.

Part 4: Modern Practice (ISO + Requirements)

Learning Objectives

  • List ISO 25010’s 9 quality characteristics
  • Apply the 6-step process for writing quality requirements
  • Write testable quality requirements using the template
  • Explain Quality in Use and Process Quality

ISO/IEC 25010:2024 — 9 Characteristics

Use ISO 25010 as a checklist when writing requirements:

# Characteristic Question It Answers
1 Functional Suitability Does it do what it should?
2 Performance Efficiency Is it fast/efficient enough?
3 Compatibility Does it work with others?
4 Usability Can users use it easily?
5 Reliability Does it keep working?
6 Security Is it protected?
7 Maintainability Can we change it?
8 Portability Can we move it?
9 Quality in Use Do users succeed?

Sub-Characteristics (Selected)

Characteristic Sub-Characteristics
Functional Suitability Completeness, Correctness, Appropriateness
Performance Efficiency Time Behavior, Resource Utilization, Capacity
Reliability Maturity, Availability, Fault Tolerance, Recoverability
Security Confidentiality, Integrity, Non-repudiation, Accountability, Authenticity
Maintainability Modularity, Reusability, Analyzability, Modifiability, Testability

6-Step Process: From ISO to Requirements

Source: Firesmith 2009

Step Action Purpose
1 Determine Relevance Which ISO characteristics matter for this project?
2 Consider Context Review functional, data, interface requirements
3 Produce Criteria Define quality criteria for each characteristic
4 Select Measures Choose how to measure each criterion
5 Specify Requirements Write testable requirements with thresholds
6 Evaluate Validate completeness, testability, feasibility

Warning: Skipping steps leads to vague requirements like “the system shall be fast.”

Worked Example: Search API Performance

Goal: Write a performance requirement for a search API

Step Action Result
1. Relevance Performance Efficiency matters for UX Selected
2. Context Users expect quick search results Search is critical
3. Criteria Response time for search queries Time-based criterion
4. Measure P95 latency (95th percentile) Measurable metric
5. Requirement Write with template Testable spec
6. Evaluate Testable? Feasible? Complete? Approved

Resulting Requirement:

While under normal load (< 1000 concurrent users), the Search API
shall respond in < 200ms, 95% of the time.

Quality Requirements Template

Template:

While in [condition], the [component] shall exhibit
[quality attribute] of [threshold] [measurement].

Examples:

Requirement Condition Attribute Threshold
API response time Normal operation Performance < 200ms, 95%
Data integrity Production Reliability 99.99%
System availability Under load Availability > 99.9%
Login authentication All users Security 0 unauthorized access

Quality in Use

Definition (ISO 25010):

The degree to which a system achieves its intended purposes when used in a specific environment by specific users.

Five Quality in Use Characteristics

Characteristic Description Example Requirement
Effectiveness Users achieve goals “90% of users complete checkout on first attempt”
Efficiency Resources used appropriately “Booking takes < 2 minutes for returning users”
Satisfaction User attitudes “NPS score > 50 in quarterly surveys”
Freedom from Risk Economic, health, environmental “No data loss during system failures”
Context Coverage Works in all intended contexts “Works on all major browsers”

Key Insight: Product quality alone is insufficient. A perfectly maintainable system that users can’t use effectively has poor quality in use.

Different Stakeholders

Need User Maintainer
Effectiveness Complete task Fix bugs
Efficiency Fast workflow Quick changes
Satisfaction Enjoys using Productive

Process Quality Characteristics

Four characteristics (Kroeger & Davidson, 2009):

Characteristic What it Measures
Suitability Does process fit the work?
Usability Can people follow it?
Manageability Can we plan, monitor, control?
Evolvability Can we improve it?

Key Insight: High quality process tends to produce high quality products (but not guaranteed!). Process quality is the focus of CMMI, ISO 9001.

Part 5: Quality Gurus

See Gurus for full details on each pioneer.

Learning Objectives

  • Identify 7 key quality pioneers and their eras
  • Match each guru to their key contribution
  • Explain key concepts: PDCA, Zero Defects, Kano Model, Juran’s Trilogy

Timeline of Quality Pioneers

Era Period Key Figures
Manufacturing Era 1920s-1950s Shewhart, Deming, Juran
Quality Movement 1960s-1980s Ishikawa, Crosby
Software Era 1980s-2000s Humphrey, Kano

Lineage: Shewhart → Deming → Juran → Humphrey

Guru Quick Reference

Guru Years One-Sentence Summary Key Contribution
Walter Shewhart 1891-1967 Control of variation PDCA Cycle, Statistical Process Control
W. Edwards Deming 1900-1993 Quality and productivity together 14 Points for Management
Joseph Juran 1904-2008 Fitness for purpose Juran’s Trilogy, Pareto Principle
Kaoru Ishikawa 1915-1989 Root cause analysis Fishbone Diagram, Quality Circles
Philip Crosby 1926-2001 Conformance to requirements Zero Defects, “Quality is Free”
Watts Humphrey 1927-2010 Father of software quality CMM/CMMI, PSP, TSP
Noriaki Kano 1940- Not all attributes are equal Kano Model

Detailed Guru Profiles

Walter Shewhart (1891-1967)

Era: Manufacturing Key Concept: Control of Variation

Contributions:

  • Statistical Process Control (SPC) — Using statistics to monitor manufacturing
  • PDCA Cycle (Plan-Do-Check-Act):
    1. Plan: Identify issues and plan solutions
    2. Do: Execute the plan
    3. Check: Compare results with planned outcomes
    4. Act: Adjust based on findings
  • Differentiated Special Cause (assignable) vs Common Cause (random) variation

Legacy: His 1924 memo at Western Electric contained “all of the essential principles… which we know today as process quality control” (George Edwards)

W. Edwards Deming (1900-1993)

Era: Manufacturing/Quality Movement Key Concept: Quality and productivity are NOT mutually exclusive

Contributions:

  • Popularized PDCA Cycle (credited to Shewhart)
  • System of Profound Knowledge:
    • Systems thinking
    • Understanding variation
    • Theory of knowledge
  • 14 Points for Management (selected):
    1. Create constancy of purpose
    2. Adopt new philosophy
    3. Cease dependence on inspection
    4. Drive out fear
    5. Remove barriers to pride
    6. Put everyone to work on transformation

Impact: Influenced Japanese manufacturing revolution

Joseph Juran (1904-2008)

Era: Manufacturing/Quality Movement Key Concept: Fitness for Purpose

Contributions:

  • Fitness for Purpose — Quality means meeting customer needs
  • Pareto Principle (80/20 Rule) — Focus on “vital few” problems
  • Juran’s Trilogy:
    1. Quality Planning — Define objectives and processes
    2. Quality Control — Monitor performance and address deviations
    3. Quality Improvement — Continuously enhance processes

Quote: “Quality means freedom from deficiencies… Higher quality in this sense usually costs less.”

Key Insight: Quality features (what customers want) and defect reduction are both important but different. You need both.

Philip Crosby (1926-2001)

Era: Quality Movement Key Concept: Conformance to Requirements

Contributions:

  • Zero Defects movement
  • “Quality is Free” (1979 book) — Quality is about conformance, not luxury
  • Quality is management responsibility
  • Price of Non-Conformance (PONC) — Measure cost of poor quality

Quote: “It is always cheaper to do the job right the first time.”

Kaoru Ishikawa (1915-1989)

Era: Quality Movement Key Concept: Root Cause Analysis

Contributions:

  • Quality Circles — Self-organized groups for problem-solving
  • Ishikawa (Fishbone) Diagram — Tool for root cause analysis
  • Seven Tools of Quality:
    1. Flowcharts
    2. Pareto diagrams
    3. Cause-effect diagrams (Fishbone)
    4. Histograms
    5. Control charts
    6. Scatter diagrams
    7. Check sheets

Noriaki Kano (1940-)

Era: Quality Movement/Software Era Key Concept: Not All Attributes Are Equal

The Kano Model [4] — Three types of quality attributes:

Type Effect Example Behavior
Must-Haves Absence = dissatisfaction Car has brakes Expected baseline
Satisfiers More = better Fuel efficiency Linear relationship
Delighters Unexpected pleasure Heated steering wheel Excitement factor

Critical Insight: Over time, delighters become must-haves!

Watts Humphrey (1927-2010)

Era: Software Era Title: Father of Software Quality

Humphrey [5] established the foundations of software process improvement.

Quote: “While software functions are most important to users, these functions are not usable unless the software runs. To get software to run, engineers must remove almost all its defects.”

Contributions:

  • CMM/CMMI — Capability Maturity Model
  • TSP — Team Software Process
  • PSP — Personal Software Process
  • Eight Steps for Quality Software:
    1. Establish quality policies and goals
    2. Train and support developers
    3. Maintain requirements quality management
    4. Apply statistical control to software processes
    5. Review all product artifacts
    6. Analyze and correct defects
    7. Manage configuration and change control
    8. Continuously improve processes

Part 6: Summary & Review

Six Key Takeaways

  1. Quality is multi-dimensional — Garvin’s 5 views help understand different perspectives

  2. Quality models provide structure — Framework for requirements, design, evaluation

  3. Models evolved over time — McCall (1977) → ISO 9126 → ISO 25010 (2024)

  4. Tradeoffs are inevitable — Cannot maximize all attributes simultaneously

  5. Quality in Use matters — User outcomes, not just product properties

  6. Gurus shaped thinking — Shewhart → Deming → Juran → Humphrey

Quick Reference Tables

Garvin’s 5 Views

View Key Word Example
Transcendent Excellence Art
Product-based Features Specs
User-based Purpose Satisfaction
Manufacturing Conformance Defects
Value-based Cost ROI

McCall’s 11 Factors (by Category)

Operation Revision Transition
Correctness Maintainability Portability
Reliability Flexibility Reusability
Efficiency Testability Interoperability
Integrity    
Usability    

ISO 25010 Characteristics (9)

Product Quality (8) Quality in Use
Functional Suitability Effectiveness
Performance Efficiency Efficiency
Compatibility Satisfaction
Usability Freedom from Risk
Reliability Context Coverage
Security  
Maintainability  
Portability  

Gurus and Their Contributions

Era Guru Key Concept
Manufacturing Shewhart PDCA, SPC
Manufacturing Deming 14 Points
Manufacturing Juran Trilogy, Pareto
Quality Movement Ishikawa Fishbone, 7 Tools
Quality Movement Crosby Zero Defects
Software Humphrey CMM, PSP, TSP
Software Kano Kano Model

References

Primary Sources:

  • Quality Views Framework [1]
  • Software Quality Factors [2]
  • Quality Attribute Relationships [3]
  • Software Design Methodology [6]
  • Software Quality Theory [7]
  • Software Process Improvement [5]
  • Kano Model [4]

Standards:

  • ISO/IEC 25010:2024 — Systems and software Quality Requirements and Evaluation (SQuaRE)
  • ISO/IEC 25000 — SQuaRE Guide
  • ISO 9126 — Software Product Quality (superseded)

SQRBOK Pages:

  1. D. A. Garvin, “What Does ‘Product Quality’ Really Mean?,” Sloan Management Review, p. 25, 1984.
  2. J. A. McCall, P. K. Richards, and G. F. Walters, “Factors in Software Quality. Volume I. Concepts and Definitions of Software Quality,” GENERAL ELECTRIC CO SUNNYVALE CA, Nov. 1977. Accessed: January 19, 2022. [Online]. Available at: https://apps.dtic.mil/sti/citations/ADA049014
  3. W. E. Perry, Effective methods of EDP quality assurance: 2nd edition. USA: QED Information Sciences, Inc., 1988.
  4. N. Kano, N. Seraku, F. Takahashi, and S. Tsuji, “Attractive quality and must-be quality,” 1984.
  5. W. S. Humphrey, Managing the software process. Addison-Wesley Longman Publishing Co., Inc., 1989.
  6. H. Zhu, Software Design Methodology: From Principles to Architectural Styles. Elsevier, 2005.
  7. A. Gillies, Software Quality: Theory and Management. Lulu.com, 2011.

Disclaimer: AI is used for text summarization, polishing and explaining. Authors have verified all facts and claims. In case of an error, feel free to file an issue.

Next Lecture: Measurement — How to quantify quality

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