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What is Interest Level (IL)?
Interest level is based on a book's content including theme, characterization, and plot. The interest level attached to the book indicates age group appropriateness. Interest level designations reflect the judgment of the book's publisher and the professionals at Renaissance Learning. We group content appropriateness into these categories: LG = Lower Grades (K-3); MG = Middle Grades (4-8); MG+ = Upper Middle Grades (6 and up); and UG = Upper Grades (9-12). Most MG+ books are marketed as YA (Young Adult) for Interest Level. However, the final decision as to whether the content of a specific book is appropriate for a particular student is the responsibility of school librarians, teachers, and parents.
What is ATOS Book Level (BL)?
ATOS Book Levels are reported using the ATOS readability formula and represent the difficulty of the text. For example, a book level of 4.5 means the text could likely be read independently by a student whose reading skills are at the level of a typical fourth grader during the fifth month of school. (Of course the content may or may not be appropriate for a fourth grader which is why we also use Interest Levels.). A range of ATOS levels is often also expressed as a ZPD range (see ZPD description).
What is a ZPD Range?
In independent literature-based reading, ZPD is the range of books that will challenge a child without causing frustration or loss of motivation. Your child will receive a ZPD range after taking a Star Reading test, or teachers and librarians can use their best professional judgment to determine a ZPD. It’s important for children to read with a high degree of comprehension and within their ZPDs. ZPDs should be adjusted based on the needs of your child. A ZPD range is synonymous with a range of ATOS levels.
What is a Lexile® measure?
A Lexile measure represents the complexity of a text, such as a book or article. Lexile measures are expressed as numeric measures followed by an “L” (for example, 850L), and are placed on the Lexile scale. The Lexile scale is a developmental scale for measuring reader ability and text complexity, ranging from below 200L for beginning readers and beginning-reader materials to above 1600L for advanced readers and materials.

Values displayed as Lexile® measures that are not followed by an “L” are estimates of Lexile text measures based on a study conducted by MetaMetrics® using ATOS Book Levels and are not certified Lexile measures.
For more information, please read "Parent's Guide to Accelerated Reader".