大量的字彙量對精通第二外語而言極為重要，由於單字量在語言教學中扮演重要的角色，因此單字量測驗有其必要，以來檢測與評估學習者在單字學習上的進步與成就。儘管如Vocabulary Levels Test和Checklist Test等英語單字量測驗的普遍使用，這些測驗對台灣的英語學習者並不是非常適當，因為這些測驗的單字所源自的字表的編製，並未考量台灣學生的學習環境和文化背景。因此本研究旨在根據大考中心所公布的6,480參考字彙，以階層測驗的形式，發展一個適當的英文單字量測驗並應用試題反應理論之三參模式（three-parameter logistic Item Response Theory model）來分析並驗證此測驗的品質。隨著測驗的建置和驗證，本研究更進一步根據三參模式估計之階層潛在難易度來探究單字頻率和潛在難度的關係，受試者包含來自台北、桃園、彰化縣的六所高中，共1060人，高一、高二、高三的人數大約均等。研究結果顯示該試題的信度可由高實證信度（0.9882）所支持，效度被反應在良好的建構效度和階級效度，此外，適配度分析（fit analysis）顯示試題品質大致而言良好，關於階層潛在難度，六個階層難度呈現非線性關係，難度從第一階層到第三階層急劇上升，從第四階層到第六階層則是逐步上升。
研究發現此研究所發展的英文單字量測驗可被視為適切的測驗，單字頻率和單字難易度並非協同地發展，此外，本研究指出階級一到階級三的單字是教師應該力推學生優先習得的基本的單字表，因為「階級三」是個轉捩點，是通往最終英文單字量目標的重要關卡。本研究期望隨著英語單字量測驗的建立及階層潛在難度的探究，教育者和學習者能洞察單字學習過程，因此能釐清如何促進單字學習目標的達成。

A substantial vocabulary size is central to mastering a second language. Due to its vital role in language teaching, vocabulary size tests are required to monitor learners’ progress and to assess their achievement in vocabulary learning. Despite the wide use of English vocabulary size tests such as Vocabulary Levels Test and the Checklist test, these tests may not be eminently suitable for English learners in Taiwan because the sources of the vocabulary in these tests are from word frequency lists compiled without considerations of the learning environment and cultural background in Taiwan. This study, then, aims to develop a proper measurement of English vocabulary size of learners in Taiwan in the form of levels test based on the 6,480 reference word list published by College Entrance Examination Center and to validate it in terms of the empirical evidence collected by using three-parameter logistic Item Response Theory Model (3PL IRT model). With the construction and validation of the test, this study further attempts to explore the relationship between word frequency and its latent difficulty based on the level latent difficulty estimated by 3PL IRT model. Participants in this study were 1,060 senior high school students from six senior high schools in Taipei, Taoyuan, and Changhua County, with the number of first, second, and third graders roughly the same. Results showed that the reliability of the test could be observed in its high empirical reliability (0.9892) and that the validity was reflected in its good construct validity and hierarchical validity. Besides, fit analysis indicated that the test items were of good quality overall. As to the level latent difficulty, it exhibits a non-linear continuum across the six levels, with a sharp rise from Level 1to Level 3 and a gradual increase from Level 3 to Level 6.
The findings suggest that the English vocabulary size test developed in this study can be considered as an adequate measurement. Word frequency does not work synergistically with its latent difficulty. Furthermore, the study reveals that words of level one to three are the fundamental and priority word list that teachers need to urge senior high school students to acquire as Level Three is a pivotal point, the door to the fulfillment of senior high school students’ ultimate English vocabulary size goals. With the development of the English vocabulary size measurement and the investigation of level latent difficulty, it is hoped that both educators and learners can gain insight into the vocabulary learning process and thus have a clear view on how to facilitate the achievement of the vocabulary learning goal.

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