Is knowing help the same as knowing helpless? A formative vocabulary assessment of Chinese and Japanese EFL learners’ knowledge of derivational forms within word families

Abstract Vocabulary instruction in EFL contexts often assumes that knowledge of a base word extends to its derivational forms, despite limited empirical evidence demonstrating this assumption across contexts. Popular formative vocabulary tests for English learners also reinforce this assumption, leaving teachers with limited insight into their learners’ knowledge of the different derivational forms within a given word family. The aim of this study was to examine EFL learners’ meaning-recall L2 vocabulary knowledge of base words and their derivational forms, and to model how a short formative assessment task can provide fine-grained diagnostic information about derivational knowledge within word families. In this study, we model the administration of a formative vocabulary assessment task among 427 tertiary-level EFL learners from China ( n = 277) and Japan ( n = 150) to evaluate knowledge of words from six high-frequency word families (i.e., help , accept , collect , develop , operate , use ). In total, learners’ knowledge of 24 words was measured, including the base word (e.g., help ) and three derivational forms (e.g., helpful , helpless , helplessness ) from each word family. From each word family, derivational forms were sampled from high (e.g., helpful ), mid (e.g., helpless ), and low (e.g., helplessness ) frequency range according to the British National Corpus (BNC). The analysis models the conditional probability that learners know a derivational form given knowledge of its corresponding base word, and examines this probability across the full sample and compares it between Chinese and Japanese learners. Within this lexical sample, higher-frequency derivational forms were generally better known than mid- and low-frequency forms, and mean conditional probabilities were lower for mid- and low-frequency derivational forms than for higher-frequency forms. Group-level comparisons further revealed differences between Chinese and Japanese learners across most derivational forms, which may reflect variation in overall proficiency and L1-specific influences.