The most effective method of testing an API program involves creating multi-step integration tests that validate common API consumer flows. API endpoints are meant to work together, so it follows that test data coming from one API that feeds another API should not be fixed or pre-built. This is very important because the less you rely on fixed data, the more unpredictable and therefore thorough the testing path will be.
Moreover, API mutation operations may have side effects that cannot be evaluated by simply validating the same endpoint. Side effects propagate throughout the system, and their efficacy can only be validated by querying other endpoints and comparing the results.