單字量跟語言學習一向有密不可分的關係，單字量在聽、說、讀、寫都扮演了重要的角色。單字的學習需要不斷地複習，要有效率地學習單字，學生需了解自己目前的單字量，才能將焦點放在符合自己程度的閱讀以及單字教材上。然而，目前並沒有針對台灣學生需求所設計的英語單字量測驗。
本研究以大考中心公布的6,840參考字彙表，設計一份英語單字量測驗，以潛在特質理論驗證此單字測驗的信度、效度以及試題品質。測驗共有180題，皆為四個選項的選擇題。並探討單字頻率與試題難度、鑑別度、猜測之間的關係。受試者為台中地區高一及高二的學生，共1,838人。
研究結果發現，由於整份試題具有良好的適合度，表示此份試題具有效度，同時也具有高信度(0.98)。高信度跟效度不只反映試題品質，同時也顯示選擇題是適合發展單字測驗的形式。單字頻率跟難度有明顯的關係，單字頻率越低，單字難度越難，而階層一到三的難度上升比階層三到六的難度劇烈。此現象顯示階層一到三的單字是學好單字的基礎，一旦學習者克服階層一到三的難關，就能夠利用大量閱讀與單字基模來學習更多的單字。另外，發現程度較差的受試者有猜答案的傾向，且偏好猜測B 與C這兩個中間選項。
從試題分析當中，發現題幹跟選項中字首字根的線索會對鑑別度造成影響，建議設計試題時需要特別注意。此現象也顯示學生有能力將單字基模中對字首字根的知識應用到新字上。

Vocabulary size is closely related to language learners’ overall proficiency. It correlates significantly with listening, speaking, reading and writing. Incorporating a word into one’s mental lexicon requires repetition. To acquire new vocabulary words effectively, a learner need to have a clear idea of his current vocabulary size to concentrate his efforts onto the reading and vocabulary learning material that fits his present vocabulary size. However, for now, no vocabulary size test is developed based on the Taiwanese learning context.
The present study is aimed to develop a vocabulary size test based on the 6,840 reference word list released by College Entrance Exam Center. The validity, reliability and test quality of the test in question are checked by the three-parameter logistic model of Latent Trait Theory. The test takes on the format of four-choice multiple choice questions, containing 180 items. The study also sets forth to explore the interaction between frequency levels and the difficulty, discrimination and pseudo-guessing parameters. Participants in this study are freshmen and sophomores from a senior high school in Taichung, totaling 1,838 subjects.
Findings revealed that the test in question exhibited satisfactory validity and reliability. Validity was supported by overall model fit and hierarchical difficulty along with the high reliability value (0.98). With validity and reliability established, the MC format was proven to be an appropriate format for vocabulary size test. Furthermore, it was found that the mean difficulty across frequency bands formed an upward nonlinear slope with the decrease of word frequency. The rise of difficulty from Level 1 to Level 3 was much steeper than that from Level 3 to Level 6. Such phenomenon revealed that the first three thousand words are the threshold for further vocabulary expansion. Once learners cross this threshold, they would be equipped with the ability to apply strategies such as extensive reading and word schema to acquire new words. On the other hand, it was found that lower-achievers are prone to take guesses and favor the distractors in the middle position. Content analysis suggests that clues in stems and distractors exert a certain influence on the discrimination power of items, which shows that learners are capable of transferring their word schema to unknown words.

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