Software Quality Models: Comparing McCall, Boehm, and ISO 9126 Standards

Foundational Software Quality Models: McCall, Boehm, and ISO 9126

McCall Software Quality Model (1977)

The McCall model breaks down the concept of quality into three capacities (or uses) important for a software product, from the standpoint of the user:

• Each use is decomposed into a number of factors that determine the quality of these capabilities. Since a factor is an abstract concept, it cannot be directly measured or evaluated.
• Each factor is decomposed into a set of criteria or basic properties, which in most cases are internal properties of software that do not depend on the observer (e.g., modularity, fault tolerance, ease of training, etc.).
• For most software developers, these criteria directly influence software quality.

Boehm Software Quality Model (1978)

The second of the predecessors of the current quality models was presented by Barry W. Boehm (1978).

• Focuses on the final product.
• Boehm attempts to define quality in a qualitative, not quantitative, way through a set of attributes and metrics.
• Like McCall, it also presents a hierarchical quality model based on 3 levels (main uses, intermediate components, and primitive features).

Comparison of Boehm and McCall

Boehm and McCall models are similar, but differ in focus:

• McCall focuses on the precise measurement of each feature at a high level.
• McCall presents a further breakdown.
• Boehm focuses more effort on a wider range (more detailed and extensive) of characteristics for maintainability, including measurements of cost and efficiency of the final product.

ISO 9126 Software Quality Standard

The objective of the standard is not necessarily perfect quality, but the necessary and sufficient quality for each context of use at the time of delivery.

History and Fundamentals

The ISO 9126 standard is built upon earlier models:

• McCall Model (1977): 3 capabilities, 11 factors, 23 criteria, 41 metrics.
• Boehm Model (1978)

Fundamentals of the standard:

• Focusing on the measurement of quality in a simple and consistent way.
• Understanding a computer system by measuring its characteristics.
• Understanding quality and attributes is a vital factor for companies.

Structure of ISO 9126

It consists of 4 parts (only Part 1 is normative):

1. Part 1: Quality Model
2. Part 2: External Metrics
3. Part 3: Internal Metrics
4. Part 4: Quality in Use Metrics