Performance testing, for example, evaluates the software's response time, throughput, and resource utilization under different load conditions. Reliability testing assesses the software's ability to maintain its performance and functionality over time. Security testing checks for vulnerabilities and weaknesses in the software's security controls. Usability testing evaluates how easy and intuitive the software is to use, while compatibility testing checks whether the software is compatible with different hardware and software configurations.

Non-functional testing is essential to ensure that the software application not only performs its intended functions but also meets the necessary non-functional requirements and provides a high-quality user experience.

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While it’s easy for non-functional requirements to be vague, proper testing demands that requirements are very specific and reasonable. A stakeholder may want the system ‘to be fast’, but testers need criteria that are measurable. Non-functional requirements need to be specific, such as: ‘the system should respond within 2 seconds during a normal load and 5 seconds during a heavy load’, with the definitions of normal and heavy loads given as well. A system could easily meet all functional requirements but still not demonstrate appropriate behaviour to an end user leading to a loss of trust in the system or a lack of desire to use it. These tests are vital to ensuring that your customers don’t look to the competition.

Examples of non-functional testing types include security testing that validates the safety of customer information, performance testing that evaluates the application and processes under varied stressors, usability testing that assesses navigation and flow, or comparison testing that places your system up against its competitors for an in-depth evaluation.