Measure of Textual Lexical Diversity (MTLD)

The Measure of Textual Lexical Diversity (MTLD) is a length-robust index of vocabulary richness introduced by Philip McCarthy in 2005 and validated by McCarthy and Jarvis in 2010. Rather than computing a single ratio over the whole text, MTLD reads the text word by word, tracking a running type-token ratio, and counts how many sequential word runs are needed before the ratio repeatedly falls to a criterion value of 0.720. The mean length of those runs, computed forward and backward and averaged, is the MTLD score — and in validation studies it was the one common index that did not vary systematically with text length.