vocd-D (D Measure)

vocd-D, also called the D measure, is a length-robust index of lexical diversity developed by David Malvern and Brian Richards. Instead of reporting a single type-token ratio, it characterizes how a text's TTR falls as sample size grows and fits that empirical curve to a one-parameter probabilistic model; the fitted parameter D is the diversity score, with higher D meaning richer vocabulary. HD-D, introduced by McCarthy and Jarvis, is the mathematically exact, sampling-free counterpart that computes the same underlying quantity directly from the hypergeometric distribution.