Someone told me Lexile is a measure of readability. It applies to books, not students. Readability is assessed by a computer algorithm. It uses length of sentences and words. I know that the F&P letter system takes everything into account including themes, and assessments are done by trained teachers 1:1.

DD once took a Lexile test for a program and scored in the 10th grade range. But her performance measured over the year showed she was really reading and comprehending at the 7th grade level.