Measures of lexical diversity in aphasia

Background: Important to the assessment of aphasia are analyses of discourse production and, in particular, lexical diversity analyses of verbal production of adults with aphasia. Previous researchers have used type-token ratio (TTR) to measure conversational vocabulary in adults with aphasia; however, this measure is known to be sensitive to sample size, requiring that only samples of equivalent length be compared. The number of different words (NDW) is another measure of lexical diversity, but it also requires input samples of equivalent length. An alternative to these measures, D, has been developed (Malvern & Richards, 1997) to address this problem. D allows for comparisons across samples of varying lengths. Aims: The first objective of the current study was to examine the relationships among three measures of productive vocabulary in discourse for adults with aphasia: TTR, NDW, and D. The second objective was to use these measures to determine in what ways, and to what degree, they each can differentiate fluent and nonfluent aphasia. Methods & Procedures: Eighteen adults with aphasia participated in this study (nine with nonfluent aphasia; nine with fluent aphasia). Participants completed the Western Aphasia Battery (WAB) and produced language samples consisting of conversation and picture description. Samples were then subjected to the three lexical diversity analyses. Outcomes & Results: Results indicated that, although the measures generally correlated with each other, adults with fluent aphasia evidenced significantly higher D and NDW values than those with nonfluent aphasia when whole samples were subjected to analyses. Once samples were truncated to 100- and 200-word samples, groups differed significantly for all three measures. Conclusions: These findings add further support to the notion that because TTR and, although to a lesser extent, NDW are sensitive to sample size, length differences across samples tend to confound results. As an alternative to these measures, the use of D for the measurement of conversational vocabulary of adults with aphasia enables the analysis of entire language samples, so that discarding language sample data is not necessary. In the present study, D values differed for fluent and nonfluent aphasia samples.