自從Palmer (1917)提出了「廣泛閱讀」的概念，此概念就在第二外語學習的領域得到相當多的關注。由於此教學法可帶來許多潛在的優點，在近幾年來便成為最重要的教學法之一。雖然在文獻中已有許多針對如何在教室內實施廣泛閱讀的討論，但是關於如何挑選閱讀材料的資訊仍略顯不足。更確切的來說，如何挑選適合某特定學生族群閱讀能力的素材，鮮少被討論。因此，為了要幫台灣國中生挑選符合其英語閱讀能力的讀本，本文採用語料庫研究的方法，針對以下幾個面向提出探討：(a)英文童書的可讀性與字彙涵蓋量 (b) 國中英語教科書的可讀性與字彙涵蓋量, 以及 (c) 此兩種文本可讀性與字彙涵蓋量的差異。分析的素材包含五套針對不同年齡層讀者的暢銷英文童書，以及兩套分別由康軒和佳音出版社所出版的國中英語教科書。分析的工具則包含用來測量可讀性的Lexile Analyzer (MetaMetrics)，以及用來探討字彙涵蓋量的RANGE Program (Nation et al., 2002, 2005)。
主要的研究結果如下：(1) 本研究所選用的英文童書的可讀性介於230L到800L之間。至於字彙涵蓋量，結果顯示在閱讀童書時，讀者需要約4,000到5,000字族 (word family) 加上專有名詞(proper nouns)跟邊際詞彙(marginal words)的知識，以達到95%的字彙涵蓋量；若需達到98%的字彙涵蓋量，讀者則需要認識7,000到9,000個字族。(2) 本研究所選用的國中英語教科書(康軒以及佳音兩種版本)的可讀性介於60L到650L之間。整體而言，佳音版本的教科書可讀性較高。此外，兩個版本的教科書皆呈現可讀性由第一冊到第六冊漸增的現象。至於字彙涵蓋量，結果顯示2,000到3,000字族加上專有名詞跟邊際詞彙的知識，即可達到95%的字彙涵蓋量；6,000個字族加上專有名詞跟邊際詞彙的知識則可達到98%的字彙涵蓋量。(3) 童書與國中英語教科書的可讀性及字彙涵蓋量比較結果顯示，本研究所選用的部分童書的難度就可讀性而言確實適合高年級國中生閱讀，然而在閱讀的過程中，字彙仍舊會是學生的一大阻礙，因為童書內約有68.91%的字族是國中英語教科書內沒有教授的。本研究結果指出使用可讀性測量工具來協助挑選閱讀材料的重要性，並點出在讓國中生獨立閱讀英文童書前，事先教授高頻率單字的必要性。

Ever since Palmer (1917) brought out the notion of “extensive reading (ER),” it has received considerable attention in the field of second language learning. Until recent years, ER has become one of the most important teaching approaches given its many potential advantages. Although discussions concerning how to carry out an ER program in a language classroom abound in the literature, little attention has been paid to the selection of reading materials, more specifically, the question about what materials are suitable for which groups of students to read is unanswered. Aiming at seeking materials that are of appropriate linguistic difficulty for Taiwanese junior high students, the present study employed a corpus-based approach to investigate the following aspects: (a) the readability and lexical coverage of children’s storybooks, (b) the readability and lexical coverage of Taiwanese junior high English textbooks, and (c) comparison between the readability and lexical coverage of the aforementioned two text types. The materials included five sets of best-selling children’s storybooks targeting at different age groups and two sets of junior high English textbooks published by Kang-hsuan and Joy respectively. Instruments used for data analysis were Lexile Analyzer (MetaMetrics) for determining readability and RANGE program (Nation et al., 2002, 2005) for investigating lexical coverage.
The major findings of the study are summarized as follows: (1) The readability of children’s storybooks selected in the study ranged from 230L to 800L. Variations in readability occurred both between and within storybook collections. As for lexical coverage, learners needed the most frequent 4,000~5,000 word families plus knowledge of proper nouns and marginal words to reach 95% coverage; whereas 7,000~9,000 word families plus proper nouns and marginal words were needed to gain 98% coverage. (2) The readability of Kang-hsuan and Joy junior high English textbooks ranged from 60L to 650L, with Joy presenting slightly higher readability level than Kang-hsuan. The readability score of each volume displayed a consistent increase from Volume 1 to Volume 6 for both versions. As for lexical coverage, the most frequent 2,000~3,000 word families plus proper nouns and marginal words were necessary to reach 95% coverage; whereas 6,000 word families plus proper nouns and marginal words were required to reach 98% coverage. (3) Comparison between the readability and lexical coverage of children’s storybooks and junior high English textbooks showed that some storybooks selected in the study were indeed readable for higher graders in junior high, but vocabulary may still pose major obstacles for them while reading because around 68.91% of the word families used in children’s storybooks were not taught in junior high English textbooks. The findings indicated the significance of using readability tools to assist reading material selections and the necessity of the pre-teaching of high-frequency words before having students read children’s storybooks independently.

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