In this study, we examined the relationship between the vocabulary size and reading comprehension of Taiwanese elementary-school graduates. We identified a lexical threshold that they need to cross to demonstrate adequate reading comprehension. We investigated the English vocabulary sizes of Taiwanese elementary-school graduates to provide solid evidence of whether the current English vocabulary-learning goal (300 words) is reasonable. We employed the Diagnosis and Certification of English Competency (DCEC) as the main assessment tool and administered the DCEC-Vocabulary Size (DCEC-VS) and DCEC-Reading Comprehension (DCEC-RC) subtests to 1903 elementary-school graduates. This study has three crucial contributions. The first contribution is evidence that explains the distribution of the vocabulary sizes of Taiwanese elementary-school graduates. The second contribution is proof of how crucial vocabulary size and lexical threshold are for an ideal curriculum. The third contribution compensates for a lack of empirical evidence regarding a lexical threshold at the elementary-school level. We propose that the application of language skills should be the main consideration when setting a lexical threshold. The degree to which specific language skills should be acquired at each level should first be proposed, followed by an exploration of the lexical threshold (minimum vocabulary size) required to match goals for specific language skills at each level. Finally, the lexical threshold proposed should be used as reference when determining the vocabulary size in the curriculum.
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