由於單字量和語言學習各方面有著緊密的關係，因此在語言學習中有著舉足輕重的地位。為了有效率地學習新字彙，學生必須先了解自己的單字量在哪個階層，才能有效地把學習重點放在符合自己程度的教材上。目前台灣學校英語學習教育多採用大考中心所公布的6,840參考字彙表，但是裡面已有部分單字對於當前台灣中小學語言學習者來說，有難度過高的狀況產生，因此本研究採用最新以COCA語料庫為基礎的字頻表，並從裡面依據字頻挑選前6,050個單字來製作單字測驗。
本測驗分A和B兩份試卷共180題，每題皆有四個選項的單選題。不同於以往的單字測驗的選字模式，本測驗只集中挑選六個單字階層中附近的字彙來製作試題，期望以是否通過某單字階層來解釋受試者的字彙量。以潛在特質理論驗證裡面的三個項度: 難易度、鑑別度、及猜測度來驗證本測驗的試題品質。受試者為台北地區的高中生及大學生，共1,198人。
根據研究結果顯示，本測驗在三個項度之中皆展現良好的適合度。由此可知此為具有高效度及信度的測驗，同時也可得知選擇題測驗是適合單字測驗的模式。字彙的難度與單字頻率有著顯著的關係，單字頻率越高，則表示其難度越低越常見。從難度上升幅度來看，階層四到六的幅度遠小於階層一到三的幅度。由此可見，單字階層三為單字學習重要的分水嶺。若學習者能融會貫通階層一到三的單字，便能應用原有的單字基模來學習更多新的單字。此外，從試題困難度分析中，題幹與選項中的詞綴會對試題困難度造成影響。因此，試題設計者須在這方面多加留意。

Vocabulary size has been considered crucial in language learning because of its strong correlation with various aspects of language acquisition. On account of its decisive status in language learning, the issue of how to learn vocabulary effectively has attracted significant attention in second language acquisition studies. Therefore, learners have to know which vocabulary size they should work toward so that they can focus on suitable learning materials. Currently, most schools in Taiwan have adopted the official CEEC wordlist, but it has been found that some of the words are too difficult for language learners. In other words, some low frequency words have been put in Level Two or Three, which hinders learners from making progress in language learning. Thus, the present study adopted the latest word frequency list from COCA and chose the most frequent 6,050 words as the sample pool to develop a vocabulary test.
The present study was divided into two forms, Form A and Form B. The format of the test is four-choice multiple choice questions. Unlike traditional vocabulary size tests, the present study only chose target words near each frequency band in order to explore the concept of passing a certain frequency level to explore learners’ vocabulary sizes. The quality of the test is checked through the difficulty, discrimination, and guessing parameters of Latent Trait Theory. The participants of the present study consisted of high school students and senior students in a national university.
According to the results of this study, this vocabulary test presents good overall model fit, which supports the validity and reliability of the present study and further indicates that the multiple choice format is suitable for a test of vocabulary size. Word difficulty has much to do with word frequency: The more frequently vocabulary occurs, the easier it is. Considering the slope of difficulty, the slope from Levels Four to Six is less steep than that from Levels One to Three, which indicates that Level Three is the cutoff point for vocabulary learning. If language learners can master the words of the first three levels, they can easily use their pre-existing word schemata to acquire more new words.
Moreover, it is noted that the influence of telltale morphemes should be taken in to consideration when analyzing the difficulty of the test. If stems are embedded so as to provide hints of corresponding word parts, caution must be taken when designing the distractors.

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