Quality Characteristics According to ISO 9126 and ISO 25010

This article is a definition of terms for quality characteristics according to ISO 9126 and ISO 25010 – including exam questions and tags.

In a Nutshell

• ISO 9126: older model with 6 main characteristics.
• ISO 25010: modernized, 8 main characteristics, more refined, adds Security and Compatibility.

Both serve the measurable specification, evaluation, and communication of software quality.

Compact Technical Description

ISO 9126 (6 Characteristics)

1. Functionality
2. Reliability
3. Usability
4. Efficiency
5. Changeability
6. Portability

ISO 25010 (8 Characteristics)

1. Functional Suitability
2. Performance Efficiency
3. Compatibility
4. Usability
5. Reliability
6. Security
7. Maintainability
8. Portability

Important shifts:

• Interoperability moves from Functionality → Compatibility
• Efficiency → Performance Efficiency (expanded to include capacity)
• Security becomes a standalone main characteristic
• Changeability → Maintainability (specifies modularity, reusability)
• Portability → Portability

Exam-Relevant Key Points

• Difference 9126 ↔ 25010 (6 → 8 characteristics, new priorities)
• Name typical sub-characteristics (Functional Completeness, Time Behaviour, Availability, Modifiability, Installability)
• Define metrics and evidence (response time, error rate, MTTR, code metrics, usability scores)
• Formulate acceptance in measurable terms (acceptance criteria per sub-characteristic)
• Recognize trade-offs (performance vs. security, maintainability vs. portability)
• Use mapping 9126 → 25010 (legacy modernization)
• Economics: quality reduces defect costs, accelerates delivery

Core Components

1. Main characteristics + sub-characteristics (both standards)
2. Mapping guide (9126 → 25010)
3. Measurement concept (targets, procedures, CI integration)
4. Evidence artifacts (requirements specification, tests, monitoring)
5. Governance (responsibilities, reviews)
6. Risks + countermeasures (e.g., caching vs. consistency)
7. Documentation (continuous traceability)

Practical Example (Web API for Order Processing)

Functional Suitability:
- Functional Completeness: all use-case endpoints present
- Functional Correctness: discount/tax calculated 100 % correctly (unit & contract tests)
- Functional Appropriateness: no unnecessary endpoints

Performance Efficiency:
- Time Behaviour: 95th percentile <200 ms with 100 users
- Resource Utilization: CPU <60 %, Memory OOM rate 0
- Capacity: 500 req/s without SLA violation

Compatibility:
- Interoperability: OpenAPI-compatible, contract tests passing
- Co-existence: runs in parallel on multi-tenant systems

Usability:
- Learnability: onboarding <30 min (task success)
- User Error Protection: validation prevents 99 % of syntactic errors
- Accessibility: WCAG AA compliant

Reliability:
- Availability: 99.9 % monthly
- Fault Tolerance: circuit breaker on third-party system failures
- Recoverability: MTTR <30 min (warm standby)

Security:
- Confidentiality: transport encryption, secrets in vault
- Integrity: signed events, checksums
- Authenticity: OIDC + MFA for admins
- Accountability: immutable audit log

Maintainability:
- Modularity: layering, no cycles
- Analysability: logs with correlations, traces
- Modifiability: feature flags, clear scope
- Testability: 70 % unit, 20 % integration, 10 % E2E

Portability:
- Adaptability: configurable tenants
- Installability: one-command deployment
- Replaceability: database exchangeable via port adapter

Advantages and Disadvantages

Advantages

• Common language about quality
• Measurable goals → better architecture decisions
• Lower defect and operational costs
• Faster acceptance

Disadvantages

• Effort for definition and measurement
• Possible goal conflicts
• Risk of bureaucratization without lived practice

Typical Exam Questions (with Brief Answers)

1. Difference ISO 9126 ↔ 25010? 25010 has 8 instead of 6 characteristics, new: Security & Compatibility, more refined sub-characteristics.
2. Where is Interoperability in 25010? Under Compatibility.
3. What does Performance Efficiency cover? Time Behaviour, Resource Utilization, Capacity.
4. Sub-characteristics of Maintainability? Modularity, Reusability, Analysability, Modifiability, Testability.
5. Example of measurable Reliability requirement? Availability 99.9 %/month, MTTR less than 30 min.

Most Important Sources

1. https://iso25000.com
2. https://en.wikipedia.org/wiki/ISO/IEC_25010
3. https://en.wikipedia.org/wiki/ISO/IEC_9126