Abstract
The choice of lexical unit has important consequences for L2 vocabulary research, testing and instruction. In recent years, the most widely used lexical unit has been the word family. This study examines the characteristics of word lists based on the word family and explores the levels of text coverage such lists may provide should the assumption that learners can deal with word families be incorrect. This is pursued through the detailed examination of a set of word-family-based word lists. The study finds that such word lists pose a number of challenges, including the number of word forms with multiple affixes, the number of word forms with more challenging affixes, and the number of word families in which the base word is not the most frequently occurring member. Moreover, the first thousand word families in particular are shown to be challenging. The study then demonstrates that if learners are unable to deal with the complexity of word families, even to a relatively small degree, word-family-based lists may provide far lower text coverage levels than may be assumed. It concludes that in work on second language vocabulary, careful consideration is needed of the appropriacy of the word family as the lexical unit and highlights the range of work based on the word family that may need reevaluating.

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