The Percentage of Words Known in a Text and Reading Comprehension

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Abstract
This study focused on the relationship between percentage of vocabulary known in a text and level of comprehension of the same text. Earlier studies have estimated the percentage of vocabulary necessary for second language learners to understand written texts as being between 95% (Laufer, 1989) and 98% (Hu & Nation, 2000). In this study, 661 participants from 8 countries completed a vocabulary measure based on words drawn from 2 texts, read the texts, and then completed a reading comprehension test for each text. The results revealed a relatively linear relationship between the percentage of vocabulary known and the degree of reading comprehension. There was no indication of a vocabulary “threshold,” where comprehension increased dramatically at a particular percentage of vocabulary knowledge. Results suggest that the 98% estimate is a more reasonable coverage target for readers of academic texts.