A Study on the Development of Grammatical Competence of Japanese EFL Learners : The Application of Item Response Theory and Differential Item Functioning

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Although the last couple of decades have seen great theoretical developments in SLA (Second Language Acquisition), many studies have focused on single grammatical features, and have assumed interrelationships among features are granted by a given linguistic theory. One of the weaknesses of grammaticality judgment task, the most common measurement tool in SLA, is its reliability. Since the data obtained from different studies are sample-and test-dependent, it is difficult to compare the findings. In order to compensate for this weakness, we employed IRT (Item Response Theory) to equate the data from different grammaticality judgment tests; since the parameters of IRT models provide a theoretical justification for equating scores, comparison with great reliability is possible of acquisition among different grammatical items on the same scale. Approximately 1,200 adult native speakers of Japanese learning EFL in Japan participated in our study. These participants were given grammaticality judgment tests in order to examine the acquisition of various English grammatical features such as unergative verbs, unaccusative verbs, psych verbs, relative clause constructions, wh-question constructions, to infinitives, and dative alternation. On the basis of our findings, we discussed the appropriateness and benefits of using IRT in SLA research, and explored interrelationships among these grammatical features in participants' acquisition process.