The most commonly used exam in schools is the achievement test. There are two types: ``norm referenced'' and ``criterion referenced.'' Either one can be a standardized test. The main difference between the two is that the score of a norm-referenced test is compared against the scores of other students. (The exam is usually designed by a testing lab and administered to thousands of students.) The score of a criterion-referenced test is measured against a certain standard of performance that indicates whether a student knows a given body of knowledge, a skill, or a process of learning.
The norm-referenced test is most often used to compare students in one school district with students from the rest of the nation. It can be used to compare one district with another. Such tests establish a ``norm'' score. A national norm line is at the 50th percentile: a score below this figure is below the national norm; a higher one is above the norm.