Use cases serve as a test basis primarily for which level of testing?

Use cases are primarily used as a test basis for system testing because they provide a detailed description of how users will interact with a system to achieve specific goals. They focus on the functionality from the user's perspective, specifying the sequence of actions and interactions. This aligns perfectly with the objectives of system testing, which aims to validate the end-to-end functionality and ensure that the software meets the specified requirements as perceived by the users.

By using use cases in system testing, testers can create test cases that cover different user scenarios, including normal usage paths and edge cases. This comprehensive approach helps identify defects and ensures that the system fulfills its intended purpose in real-world usage.

While other testing levels such as unit testing or load and performance testing have their specific focuses and methodologies, they do not primarily leverage use cases in the same manner as system testing. Unit tests typically evaluate individual components for correctness without considering user interactions, while load and performance tests focus on the system's behavior under stress rather than functional flows described in use cases. Usability testing, on the other hand, emphasizes user interface and experience rather than functional correctness based on user interactions outlined in use cases.