The concept of reading level is to find reading materials that are appropriate for a child to read independently and not be overwhelmed or bored. Taking topics, concepts, and vocabulary into account books are leveled by professionals in the education field to help both parents and teachers find appropriate books for each student to become successful. Reading level is not necessarily tied to the grade level, you may have a 2nd grader who is a book worm and reading books well above their grade, or conversely, you could have a 6th grader who is a struggling reader reading below level.
The Lexile Framework® for Reading is a scientific approach to measuring both reading ability and the text complexity of reading materials on the same developmental scale.
Educators use Lexile measures to match students with text, putting them on the path to success in school, college and careers.
Fountas and Pinnell created the Guided Reading Level. This leveled reading system assigns a level to individual books. Factors that impact that level include repetition of words, the complexity of sentences, and sentence length.
The Developmental Reading Assessment, or DRA, assesses a child's reading ability through a reading test. It then gives the student a score based on that test. Factors it checks include:
Accelerated Reader is a complex readability formula that rates text complexity. It uses grade level ratings to indicate what year and month a student could read a book independently.
Accelerated Reader is different from straight grade level equivalents because it uses interest level as well. This indicates if the content of a book is age-appropriate for the particular grade level.