As a more concrete example, suppose that an instructor evaluates a group of students. In this situation, the underlying variable is knowledge or master of subject matter, and the instructor’s goal is to obtain a measure of known edge for each student. However, it is impossible for the instructor to look inside each student’s head to measure how much knowledge is there. Therefore instructors typically give students a task (such as exam, an essay, or a set of problems), then measure how well students perform the task. Although it makes sense to expect that performance is a reflection of knowledge, performance and knowledge are not the same thing. For example, physical illness or fatigue may affect performance on an exam, but they probably do not affect knowledge. There is not a one-tone relationship between the variable that the instructor wants to measure (knowledge) and the actual measurements
that are made (performance).