大家都知道,Cronbach's alpha受到幾個因素的影響,其中之一就是問項間的相關係數(interitem correlation)高低。除了這個以外,當然也跟題目的多寡、以及樣本的大小有關。
但是interitem correlation要多高才叫夠呢?如果不回答這個問題,大概也很難決定為什麼alpha會很低吧。
剛剛查了一下文獻,Hulin et al. (2001)裡面有提供一些建議(p.57):
Robinson et al. (1991) advocated an alpha level of .80 or better and average interitem correlations of .30 or better as exemplary. Clark and Watson (1995) advocated average interitem correlations of .15 to .50 across constructs, and for narrowly defined constructs, they advocated a range of .40 to .50 for average interitem correlations.
所以啦,根據這些文獻的建議,大概就是.3或是更高一些,才是足夠的interitem correlation。變數的定義範圍如果窄一些,需要的interitem correlation也就比較高(很合理啊)。
不過必須提醒讀者的是,這些數值僅供參考。如果讀者參閱Schmitt (1996)的paper,不難會發現.3的interitem correlation在Schmitt的模擬中被視為是low interitem correlation...
參考文獻:
Clark, Lee Anna, & Watson, David. (1995). Constructing validity: Basic issues in scale development . Psychological Assessment, 7, 309–319.
Hulin, C., Netemeyer, R., & Cudeck, R. (2001). Can a Reliability Coefficient Be Too High? Journal of Consumer Psychology, 10, 55-58.
Robinson, John P., Shaver, Phillip R., & Wrightsman, Lawrence S. (1991). Criteria for scale selection and evaluation. In John P. Robinson, Phillip R. Shaver, & Lawrence S. Wrightsman (Eds.), Measures of personality and social psychological attitudes (pp. 1–15). San Diego, CA: Academic.
Schmitt, N. (1996). Uses and Abuses of Coefficient Alpha. Psychological Assessment, 8, 350-353.