Table 2.

Definition of surface metrics and readability measures

Surface Metrics and Readability Measures	Definition
Average Document Length by Sentence (ADL-SENT)	Average number of sentences per document
Average Document Length by Lexicon (ADL-LEX)	Average number of lexicons per document
Vocabulary Coverage (VC)	Number of words covered by a dictionary normalized by the vocabulary size (percentage)
Flesch-Kincaid Grade Level (FKGL)	0.39 * (number of lexicons / number of sentences) + 11.8 (number of syllables / number of lexcons) 15.59; a raw score can be rounded to the nearest integer below its current value to form a grade level.
Gunning-Fog Index (GFI)	0.40 * [(number of lexicons / number of sentences) + 100 * (number of difficult words / total lexicons)]; a word is considered difficulty if it has 3 or more syllables.
Average Readability Score (ARS)	The average of the raw scores of FKGL and GFI. Similarly, an ARS score can be rounded to the nearest integer below its current value to form a grade level.