If a person were to ask an educator to identify the two most important attributes of an education test, the response most certainly would be "validity and reliability." These two tightly wedded concepts have become icons in the field of education assessment. As far as validity is concerned, the term doesn't refer to the accuracy of a test. Rather, it refers to the accuracy of score-based inferences about test takers. Once educators grasp the idea that these inferences are made by people who can, of course, make mistakes, they're apt to be more cautious about how to use test results. In the case of reliability, however, it's the test itself that is or isn't reliable. That's a whopping difference. One would think, therefore, that most educators would have a better handle on the meaning of reliability. Unfortunately, that's not the case. In this article, the author explores the definition of reliability. The author then explains a situation in which teachers actually need to know what's going on regarding reliability.