If one class scores high and another scores low, what might the test not be?

The focus of this question is on the concept of reliability in testing. Reliability refers to the consistency of a test in measuring what it's intended to measure. If one class scores significantly high and another class scores significantly low on the same assessment, it suggests that the test may not consistently measure the intended knowledge or skills across different groups.

In a reliable test, one would expect that if the students have similar levels of knowledge or skills, their scores would be similar, regardless of which class they belong to. A large disparity in scores indicates that the test might be influenced by factors such as the test format, the nature of the questions, or the subject matter being assessed, rather than the actual abilities of the students. Therefore, a reliable test should produce stable and consistent results across different populations, demonstrating that it effectively measures the same underlying construct for all groups.

Thus, if the scores differ drastically between classes, it is indicative of a potential issue with reliability, signaling that the assessment may not perform uniformly across varying groups.