1) Achievement tests measure students' knowledge and skills in a particular subject area that has been taught. There are two main types: standardized tests which are widely administered and informal classroom tests constructed by teachers.
2) Standardized achievement tests (SATs) are carefully developed by experts, pre-tested on large groups, and administered and scored uniformly according to strict guidelines. They provide norms for comparing students' performance.
3) SATs measure many areas and can be used for diagnostic, competency, or predictive purposes. Formal batteries allow comparison across subjects. Informal tests have more variable quality.