Investigating accommodation in language proficiency interviews using a new measure of lexical diversity

Lexical diversity is an important indicator of language learners’ active vocabulary and how it is deployed. Traditionally it has been measured by the Type-Token Ratio (TTR), the ratio of different words to total words used. Unfortunately, TTR is a function of sample size: larger samples of words will give a lower TTR and even commonly used measures derived from TTR which are claimed to be independent of sample size are problematic. To overcome this, the authors have developed an innovative measure of vocabulary diversity, D, based on mathematically modelling how new words are introduced into larger and larger language samples, and have produced software (vocd) to calculate it. Previous research by the authors into language proficiency interviews (Richards and Malvern, 2000) investigated linguistic and discourse accommodation of teacher-testers using a wide range of student and teacher variables. In a study of teenage learners of French, the aspect of teachers’ language in oral interviews that was most responsive to the ability of their students was lexical diversity. The analysis reported here focuses on this finding in greater depth using the new measure, D. The relationship between D and other measures of foreign language proficiency is investigated, the Ds of students and teachers are compared and the correlations between teachers’ D and students’ proficiency are computed. Results firstly demonstrate the validity of D as a measure of vocabulary diversity and the effectiveness of vocd as a tool to analyse language data. Secondly, with regard to accommodation processes in oral testing, the two teachers did not finely tune their vocabulary diversity to the proficiency of individual students. Instead, each teacher roughly adjusted his or her language to the ability of the class they examined.