What is a Lexile Measure?
"Lexile Measure" refers to one of two things: a text's difficulty or a reader's proficiency. All Lexile Measures are represented as a series of numbers followed by an L, e.g., 1080L.
Text Measures
When applied to a text, Lexile Measures tell you how difficult the text is. They cannot tell you whether the text contains meaningful or valuable content; the Lexile Measure is strictly a measure of difficulty.
Some books with low Lexile Measures contain challenging ideas that are tough to understand, and vice-versa. Just because a text is easy to comprehend doesn't mean that your students will understand the ideas the text presents.
Reader Measures
Applied to a reader, Lexile Measures are a bit more complicated.
First, student measures are determined by scores on end-of-year reading assessments. Not all states report Lexile Measures from these assessments; you can see if your state is part of the program here.
Second, if a student's Lexile Measure exactly matches that of the text they're reading, that student can be expected to comprehend approximately 75% of the text.
As a student's Lexile Measure increases, the percentage of the text they can be expected to comprehend increases as well. However, this increase is not on a linear scale; with an increase of 250L, the student can be expected to comprehend 90% of a text (a gain of 15 percentage points). With an increase of an additional 250L, the reader can be expected to comprehend 96% of a text (a gain of only 6 additional percentage points).
As a student's Lexile Measure decreases, however, comprehension drops off quickly. Each decrease of 250L chops 25 percentage points off the expected comprehension rate.
For this reason, it's inadvisable to assign students texts with Lexile Measures very far above their personal Measures; best practice is to assign texts no higher than 50L above a student's personal Measure.
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If you have questions about Lexile Measures that aren't answered in this post, email us, and we'll find the answers. Be sure to put "Lexile" in the subject line so we can find your message!
