Scholastic Reading Inventory scores require the use of a mathematical formula to determine the difficulty of text selections in an item bank. A number of these text selections are presented to a student, and she is asked a comprehension question about each passage she reads. A Lexile score is then assigned to the student based on the average difficulty of text she was able to comprehend.
A number of methods exist for determining the difficulty of a reading passage, but most take into account vocabulary, sentence length, and sentence structure. The Lexile scale was created by choosing a high point, the Electronic Encyclopedia, and a low point, seven basal primers, and using one of these methods to score the selections and create a range between them. Grade level ranges were then identified by assessing the levels of textbooks across the nation and by plotting the growth a student needs in order to be ready to read college-level text.
The SRI begins by giving a student an on-level passage and asking her to fill in a sentence related to what she read. If the student answers correctly, she is given a slightly more difficult passage, and vice-versa if she answers incorrectly. The test continues to select passages that are more or less difficult than the previous selection until it can determine an average range in which the student can read and comprehend.