Modelling vocabulary growth in a foreign language
Daller, M., Turlik, J. and Weir, I. (2012) Modelling vocabulary growth in a foreign language. Under consideration for publication in The Validity of Vocabulary Measures. [Submitted] Available from: http://eprints.uwe.ac.uk/16293
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Many studies in a variety of educational contexts show that learning curves are non-linear (e.g. Freedman 1987 for the development of story telling skills in the first language, De Keyser 1997 for the acquisition of morphosyntactic rules of an artificial second language or Brooks and Meltzoff 2007 for the development of vocabulary in two year old infants) but there is no agreement on the best non-linear model which may vary between different contexts. Although there are strong arguments, both on empirical and on theoretical grounds, that a power curve is appropriate in most educational settings (Newell and Rosenbloom 1984, Ninio 2007) other models have also been proposed (Van de gaer et al. 2009, Verhoeven and Van Leeuwe 2009). However, little is known about the long-term patterns of vocabulary learning in a foreign language. In the present study we analyse the vocabulary used in 294 essays of 42 students written over a period of two years. We use several measures that focus on vocabulary richness and ratings from trained IELTS teachers. Our analysis is supported with structural equation modelling, where a latent learning curve, based on the power law, can be identified. The present study is relevant for the discussion on methodological approaches in the measurement of vocabulary knowledge but also has pedagogical implications, as it allows teachers to identify when a certain plateau has been reached and further vocabulary learning is only effective with additional pedagogical interventions.