本文從試題特徵曲線著手，說明試題反應理論有關「概率」、「能力或特質量尺 」的觀念。接著以不同試題特徵曲線，比較說明「單參數」、「雙參數」、「三參數」模式 函數描述與圖型之相異處。最後再用圖型說明試題反應理論的二個基本假定：單維性、局部 獨立性。

　　　　　This article started from item characteristic curve to explain the relationship between probability and ability /trait scale relating to item response theory. With the different item characteristic curve, we compared the differences of figures among mathematical forms of one-parameter logistic model 、 two-parameter logistic model and three-parameter logistic model. Finally, we took advantage of figures to explain two basic assumptions of item response theory: unidimensionality、local independence.