國中基測英語科雙峰分配探索

Exploring the Bimodality of the Score Distribution of the English Test of the Basic Competence Test for Junior High School Students

國中基測英語科的雙峰分配長久以來一直受到各界的關心和研究，本文的重點不在探討社會影響或教育政策，我們的重點將擺在利用現代測驗統計技術，去探索雙峰分配背後的特性和成因。我們首先驗證雙峰不是試題特性影響所造成的，再驗證左側峰很可能是隨機猜測造成的，最後利用答對題數分配的模擬，得到的結果如同我們推論的假設，雙峰的成因來自於過大的能力變異和英語出題的門檻，造成一大半的人覺得題目很簡單，形成負偏態；另一大半的人卻覺得題目很難，形成正偏態，而這些人又表現出猜測行為，最後就形成我們所看到的古典分數的雙峰分配。

The bimodal score distribution of the test of English of the Basic Competence Test for Junior High School Students (BCTEST) has received great research interests since 2001. In this article， instead of discussing the social and educational issues， we applied psychometric and statistical methods to reconstruct the score distribution and exploredthe reasons and characters of the bimodality of the score distribution of the English test.It was found that assuming a normal ability distribution test items calibrated using different Item Response Theory models did not yield a bimodal score distribution. Second，it was demonstrated that the left-end mode might be caused by examinees’ guesswork.Finally， the bimodal score distribution may be caused by the large ability variance andthe threshold of test difficulty made by the English form item.