An important gap in the field of second language vocabulary research concerns the ability of Asian learners of English as a Foreign Language (EFL) to comprehend inflectional and derivational word family members. Japanese EFL learners (N = 279) were divided into three lexical proficiency groups, and their ability to comprehend inflectional and derivational English forms was measured with an English to Japanese translation test. A significant difference among the participants' ability to comprehend 12 base forms, associated inflected forms, and associated derived forms was found across the three proficiency groups, and even among participants who demonstrated mastery of the first 4,000 or 5,000 base forms of English. The flemma, a word's base form and associated inflectional forms, was found to be an appropriate word counting unit for most participants. Results are important because corpus research findings demonstrate that in cases where the word family provides 98 per cent coverage of texts, the flemma only provides 85 per cent coverage of the same texts. Thus, considering the detrimental impact to reading comprehension from only small decreases in the percentage of known tokens within a text, the results question the inferences made in word family-based research.

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