International Software Testing Qualifications Board (ISTQB) Foundation Level Practice Test

Usability testing is categorized as non-functional testing because it evaluates how user-friendly and accessible a software application is, rather than focusing on whether the software produces the correct outputs under specified conditions. Non-functional testing assesses the attributes of the system, such as performance, reliability, and usability, which are essential for the overall user experience.

In usability testing, the key objective is to determine how well users can interact with the software and whether it meets their needs in terms of efficiency, effectiveness, and satisfaction. This involves observing real users while they perform tasks, gathering qualitative feedback, and analyzing metrics related to user experience.

Functional testing, on the other hand, concentrates on verifying that the software functions according to specified requirements and meets the expected outputs. Structural testing focuses on the internal workings of the application, such as code coverage and path testing, rather than the user experience. Change-related testing assesses the impact of changes made to the software on existing functionality and features.

Therefore, usability testing's goal of measuring the interface's ease of use and overall user satisfaction firmly positions it within the realm of non-functional testing.