Studies on reading in a foreign language report that meaningful reading comprehension is determined by an individual's vocabulary size. A conclusion from these studies is that a vocabulary threshold of 8000–9000 words is compulsory for the reader to comprehend 98% of running words in academic texts and consequently achieve optimal comprehension of the materials. A threshold of 4,000–5,000 words can only assist readers to understand 95% of running words, which guarantees minimal comprehension. This study examines the relationship between the vocabulary size of undergraduate university students and the vocabulary coverage of reference books sampled from among those listed in their course outlines. This corpus-based and descriptive study used the Vocabulary Level Test (VLT) to assess the vocabulary size of 774 participants and a vocab profiler to analyze the vocabulary coverage of the nine sampled reference books. The results showed that, on average, the participants’ vocabulary size would enable them to comprehend 95% of the running words in most of the books in the sample, but that size would not help them to understand 98% of the running words in any of the sampled books. These results suggest that, on average, no student in the sample could have optimal comprehension of the sampled reference books. Therefore, our study calls for the need to introduce serious reading programmes at the primary and secondary school levels so as to promote students' vocabulary size and reading comprehension ability.
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