How well do learners know derived words in a second language? The effect of proficiency, word frequency and type of affix

The study investigates derivational knowledge of second language (L2) learners as a function of four variables: learner proficiency, word family frequency, derived word frequency, and affix type as suggested by two affix difficulty hierarchies. Seventy-nine EFL learners at two proficiency levels received two tests, the VST - Vocabulary Size Test (Nation & Beglar, 2007) and a custom-made `Derivatives Test', which included derived forms of VST base words. We performed the following within-participant comparisons: knowledge of base words and knowledge of their derived forms, knowledge of derived forms from high-, medium, and low-frequency word families and knowledge of derivatives at different affix difficulty levels. Knowledge of basewords and their derivatives was statistically equivalent for advanced learners. However, a difference was found between the categories for less advanced learners. The findings also revealed learner proficiency and base word frequency effects, partial support for the two affix difficulty hierarchies, and no support for the effect of derivative frequency.