Validation vs. Verification

Validation vs. Verification

(‘Serendipity’, November 29, 2010)

The terms Verification and Validation are commonly used in software engineering to mean two different types of analysis. The usual definitions are:

Validation: Are we building the right system?
Verification: Are we building the system right?

In other words, validation is concerned with checking that the system will meet the customer’s actual needs, while verification is concerned with whether the system is well-engineered, error-free, and so on. Verification will help to determine whether the software is of high quality, but it will not ensure that the system is useful.

The distinction between the two terms is largely to do with the role of specifications. Validation is the process of checking whether the specification captures the customer’s needs, while verification is the process of checking that the software meets the specification.

Verification includes all the activities associated with the producing high quality software: testing, inspection, design analysis, specification analysis, and so on. It is a relatively objective process, in that if the various products and documents are expressed precisely enough, no subjective judgements should be needed in order to verify software.

In contrast, validation is an extremely subjective process. It involves making subjective assessments of how well the (proposed) system addresses a real-world need. Validation includes activities such as requirements modelling, prototyping and user evaluation.

In a traditional phased software lifecycle, verification is often taken to mean checking that the products of each phase satisfy the requirements of the previous phase. Validation is relegated to just the beginning and ending of the project: requirements analysis and acceptance testing. This view is common in many software engineering textbooks, and is misguided. It assumes that the customer’s requirements can be captured completely at the start of a project, and that those requirements will not change while the software is being developed. In practice, the requirements change throughout a project, partly in reaction to the project itself: the development of new software makes new things possible. Therefore both validation and verification are needed throughout the lifecycle.

Finally, V&V is now regarded as a coherent discipline: ”Software V&V is a systems engineering discipline which evaluates the software in a systems context, relative to all system elements of hardware, users, and other software”. (from Software Verification and Validation: Its Role in Computer Assurance and Its Relationship with Software Project Management Standards, by Dolores R. Wallace and Roger U. Fujii, NIST Special Publication 500-165)