During which test activity could faults be found most cost-effectively?

The most cost-effective stage to find faults is during the planning phase. This phase involves defining the scope and objectives of the testing process, as well as identifying potential risks and their associated mitigation strategies. By addressing issues and uncertainties early in the planning stage, testers can develop a clear understanding of requirements and expectations, which helps avert faults that could lead to more extensive problems if discovered later.

Finding bugs during the planning phase is typically less expensive than during execution, design, or when checking exit criteria because it minimizes the time and resources needed for rework. Catching problems early prevents them from evolving into larger issues, which would require more intensive effort and funding to resolve later in the project lifecycle. Additionally, it allows for clearer test case creation and test environment set-up based on well-defined requirements, facilitating a smoother testing process.

In contrast, while faults can also be identified in execution and design activities, these phases often involve more complex scenarios where rectifying faults can require significant changes to code or test cases, typically incurring higher costs. Checking exit criteria is crucial but is focused on verifying that tests meet predefined conditions, which may not uncover faults as cost-effectively as addressing them in the planning phase.