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All books have a certain level of complexity that is often referred to as a "reading level." Generically speaking, reading levels are typically defined using some formula involving frequency of common words, number of syllables per word, syllables per sentence, and words per sentence. Vocabulary is sometimes considered, as well. Teachers can, in theory, apply one of these formulas to a passage of text, calculate the level of difficulty, and thus gain a clear understanding of what students would be successful at reading the book.
A reading level score on a text can help everyone know how difficult the text will be and when the reading should appropriately be assigned. It can help students self-select appropriate literature, as well. Unfortunately, it's simply not that easy or clear-cut. There are at least five different kinds of reading leveling systems in common use, and there is no simple conversion between them. Here we're specifically referencing Accelerated Reader (AR) programs and also Developmental Reading Assessment (DRA) levels.
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Recognition and Assessment
Depending on the assessment tool and reading curriculum used in your program and the publisher of the book you are considering, it's very possible that the reading levels you need to match will be in different languages. If a reading level is noted by a letter code, it might be in the Fountas/Pinell System that runs from A through V or the Scholastic Guided Reading Level which is from A through Z. DRA (Developmental Reading Assessment) levels begin at Level A and then 1 for early kindergarten readiness and go through 80 by grade 6. Lexile Levels begin at 200 for kindergarten and rise to 1050 by grade 6.
Renaissance Learning (the company that created the Accelerated Reader Program) uses a system called the ATOS scale, that looks a lot like grade levels (starting at .4 in kindergarten and rising to 12.0 in twelfth grade). It's important to find out what reading difficulty scale is being used by your assessment tool, your reading curriculum and the book's publisher.
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Converting Between Reading Level Systems
Many reading programs now encourage using trade books to supplement the curriculum's provided lessons and texts. This is easily done if both your reading program and your trade books have clear reading level designations within the same system, but how can you manage when the leveling system of the curriculum does not match the leveling system used by the publisher?
One method of conversion is to compare many books that have been leveled with both systems for which you need conversions. Make a chart about which levels match, put the information on a graph, then do some curve-fitting to fill in the gaps. This does take quite a bit of hands-on research and some skill with statistics.
Alternatively, several websites have published charts that list conversions between various systems, including the DRA (Developmental Reading Assessment) and the ATOS Scale (used by Accelerated Reader). You can find conversion charts on the following websites: