Software testability

Software testability is the degree to which a software artifact (i.e. a software system, software module, requirements- or design document) supports testing in a given test context.

Testability is not an intrinsic property of a software artifact and can not be measured directly (such as software size). Instead testability is an extrinsic property which results from interdependency of the software to be tested and the test goals, test methods used, and test resources (i.e., the test context).

A lower degree of testability results in increased test effort. In extreme cases a lack of testability may hinder testing parts of the software or software requirements at all.

Background

The effort and effectiveness of software tests depends on numerous factors including:

• properties of the software requirements
• properties of the software itself (such as size, complexity and testability)
• properties of the test methods used
• properties of the development- and testing processes
• qualification and motivation of the persons involved in the test process

Testability of Software Components

The testability of software components (modules, classes) is determined by factors such as:

• controllability: The degree to which it is possible to control the state of the component under test (CUT) as required for testing.
• observability: The degree to which it is possible to observe (intermediate and final) test results.
• isolateability: The degree to which the component under test (CUT) can be tested in isolation.
• separation of concerns: The degree to which the component under test has a single, well defined responsibility.
• understandability: The degree to which the component under test is documented or self-explaining.
• automatability: The degree to which it is possible to automate testing of the component under test.
• heterogeneity: The degree to which the use of diverse technologies requires to use diverse test methods and tools in parallel.

The testability of software components can be improved by:

• Test-driven development
• design for testability (similar to design for test in the hardware domain)

Testability of Requirements

Requirements need to fulfill the following criteria in order to be testable:

• consistent
• complete
• unambiguous
• quantitative (a requirement like "fast response time" can not be verified)
• verifiable in practice (a test is feasible not only in theory but also in practice with limited resources)