title: Psychometric Evaluation of Lexical Diversity Indices: Assessing Length Effects.
abstract: OBJECTIVE
  Several novel techniques have been developed recently to assess the breadth of a 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, and moving-average type-token ratio.
  
  
  METHODS
  Four LD indices were estimated for language samples on 4 discourse tasks (procedures, eventcasts, story retell, and recounts) from 442 adults who are neurologically intact. The resulting data were analyzed using structural equation modeling.
  
  
  RESULTS
  The scores for measure of textual lexical diversity and moving-average type-token ratio were stronger indicators of the LD of the language samples. The results for the other 2 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 4 estimation techniques and should assist clinicians and researchers in the selection of LD measures of language samples that minimize construct-irrelevant sources.
authors.forename: Gerasimos
authors.initials: G
authors.lastname: Fergadiotis
authors.forename: Heather Harris
authors.initials: HH
authors.lastname: Wright
authors.forename: Samuel B
authors.initials: SB
authors.lastname: Green
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D005260:Female; D006801:Humans; D007807:Language Tests; D008297:Male; D008875:Middle Aged; D015233:Models, Statistical; D011594:Psychometrics / Q000379:methods; D014825:Vocabulary; D055815:Young Adult