The online dictionary Dictionary.com defines the word "diagnostic" as an adjective meaning "serving to identify or characterize; being a precise indication." Diagnostic testing in math is done to assess students' abilities and skill levels.
Diagnostic tests for ability in pre-algebraic math concepts are usually administered to students 10 years and older, up through eighth grade. These tests help assess whether the student is ready to move on to study algebra.
Each math diagnostic test covers a specific concept. The subject matters include whole numbers, fractions, word problems, metrics, percentages and graphs.
Math diagnostic testing can detect a student's learning deficiencies that the teacher may not have identified. Administering identical, standard diagnostic math tests creates a consistent and fair environment in which to evaluate student progress.
Some schools use diagnostic math tests to determine in which math classes to place students. It is detrimental to a student to be placed in a class for which she is not ready. It is equally detrimental to not let a student progress when ready.