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Building an English Vocabulary Knowledge Dataset of Japanese English-as-a-Second-Language Learners Using Crowdsourcing

We introduce a freely available dataset for analyzing the English vocabulary of English-as-a-second language (ESL) learners. While ESL vocabulary tests have been extensively studied, few of the results have been made public. This is probably because 1) most of the tests are used to grade test takers, i.e., placement tests; thus, they are treated as private information that should not be leaked, and 2) the primary focus of most language-educators is how to measure their students' ESL vocabulary, rather than the test results of the other test takers. However, to build and evaluate systems to support language learners, we need a dataset that records the learners' vocabulary. Our dataset meets this need. It contains the results of the vocabulary size test, a well-studied English vocabulary test, by one hundred test takers hired via crowdsourcing. Unlike high-stakes testing, the test takers of our dataset were not motivated to cheat on the tests to obtain high scores. This setting is similar to that of typical language-learning support systems. Brief test-theory analysis on the dataset showed an excellent test reliability of $0.91$ (Chronbach's alpha). Analysis using item response theory also indicates that the test is reliable and successfully measures the vocabulary ability of language learners. We also measured how well the responses from the learners can be predicted with high accuracy using machine-learning methods.