Purpose—Several novel techniques have been developed recently to assess the breadth of speaker’s vocabulary exhibited in a language sample. The specific aim of this study was to increase our understanding of the validity of the scores generated by different lexical diversity (LD) estimation techniques. Four techniques were explored: D, Maas, Measure of Textual Lexical Diversity (MTLD), and the Moving Average Type Token Ratio (MATTR).

Method—Four LD indices were estimated for language samples on four discourse tasks (procedures, eventcasts, story retell, and recounts) from 442 neurologically intact adults. The resulting data were analyzed using structural equation modeling.

Results—The scores on the MATTR and MTLD estimation techniques were stronger indicators of the LD of the language samples. The results for the other two techniques were consistent with the presence of method factors representing construct-irrelevant sources.

Conclusions—These findings offer a deeper understanding of the relative validity of the four estimation techniques and should assist clinicians and researchers in the selection of LD measures of language samples that minimize construct-irrelevant sources.