Test scores reduce complex phenomena, such as achievement in mathematics or ability to comprehend a written text, to numbers. If a test has done a good job of testing what it was designed to assess, then test results can be useful, especially when we want to do things like track progress and make comparisons. However, it is important to understand that test scores are not error free. This Assessment News article is the first of two that provide a “quick guide” to some sources of error that can make our use of test results problematic and the implications this has for practice. In this first article, we look at the impact of measurement error on results for individuals and groups.