What is a common characteristic of a well-designed diagnostic test?

A well-designed diagnostic test is characterized by having clear instructions and measurable outcomes. This clarity is crucial because it ensures that students understand what is expected of them and can navigate the test without confusion. Moreover, measurable outcomes allow educators to assess students' understanding, pinpoint areas of strength, and identify aspects that require further instruction or intervention. This structured approach helps both teachers and students focus on the essential skills and knowledge that need to be developed.

The other provided options focus on aspects that may not contribute to a diagnostic test's effectiveness in education. For example, assessing only high-level thinking skills would neglect the importance of foundational knowledge and skills necessary for learning. A test designed to challenge students must strike a balance and not create undue difficulty, which could discourage participation. Lastly, while a test that can be taken without any preparation may sound appealing, it doesn't allow for accurate measurement of student progress or readiness; preparation is often part of the learning process that diagnostic tests aim to evaluate.