Chapter 7. Vocabulary acquisition and the learning curve
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, DeKeyser, 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 & Rosenbloom, 1981; Ninio 2007) other models have also been proposed (Van de gaer et al., 2009; Verhoeven & 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 by 42 students written at regular intervals over a period of two years. We use several measures that focus on vocabulary richness as well as 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 when further vocabulary learning is only effective with additional pedagogical intervention.