How are the reading levels of children's books decided?

There are three different types of reading levels used to categorize children's books. The Developmental Studies Center (DSC) reading level, Guided reading level, and Lexile reading level area, each decided in different ways.

The DSC reading level is determined by journal reviews, classroom experience, and student response. It also takes the Guided and Lexile reading levels into account.

The Guided reading level is based on qualitative assessments by book publishers, distributors, and library databases.

The Lexile reading level determines a book's difficulty by qualitatively assessing slices of its text. The Lexile system also takes the difficulty of reading the back cover into account.