Ph.D. Student Fred Zenker and colleagues who were formerly at 糖心Vlog官方M recently wrote a paper that was released in Studies in Second Language Acquisition.
Title: Evaluating evidence for the reliability and validity of lexical diversity indices in L2 oral task responses
Abstract:
Although lexical diversity is often used as a measure of productive proficiency (e.g., as an aspect of lexical complexity) in SLA studies involving oral tasks, relatively little research has been conducted to support the reliability and/or validity of these indices in spoken contexts. Furthermore, SLA researchers commonly use indices of lexical diversity such as Root TTR (Guiraud鈥檚 index) and聽D聽(vocd-D and HD-D) that have been preliminarily shown to lack reliability in spoken L2 contexts and/or have been consistently shown to lack reliability in written L2 contexts. In this study, we empirically evaluate lexical diversity indices with respect to two aspects of reliability (text-length independence and across-task stability) and one aspect of validity (relationship with proficiency scores). The results indicated that neither Root TTR nor聽D聽is reliable across different text lengths. However, support for the reliability and validity of optimized versions of MATTR and MTLD was found.