Assessment, Statistics, and Research

Why aren't tests reliable? The reason tests are not reliable is that reliability is a property of the interpretation of scores not the tests themselves.   This isn't a matter of semantics. Think about it this way. Give all the students in one school an achievement test. The test items don't change so they appear stable, consistent, and reliable. However, when publishers report reliability values, they calculate the reliability statistics based on scores. Scores vary from one administration to another. If you ever took a test twice and got a different score, you know what I mean. Individuals change from day to day. And we change from year to year. Also, even a representative sample of students for a nation can be different each year. Every time we calculate a reliability statistic, the statistic is slightly different. Reliability values vary with the sample. Reliability values vary with the method of calculation. Reliability values also vary with the method us...