When to calculate Cronbach's alpha for a paper?

Question

I am doing my undergraduate thesis, and I wonder if I need to do the Cronbach's alpha from my data set, or if I should use the Cronbach's alpha from the commercial questionnaire I will use for this paper. What is the standard way to approach this?

Answer 1

The standard practice—at least in psychology—is to calculate and report a Cronbach's alpha for any scale that you create.

Say that you have a commercial questionnaire that is four items long: Item 1, Item 2, Item 3, and Item 4. The standard approach is to simply average the participant's responses across those items. When we do that, we often report a Cronbach's alpha as a quick-and-dirty measure of reliability.

Here is an example from a paper I co-authored:

We employed the same free speech items as in Studies 4 through 6 ($\alpha = $ .97), and used the symbolic racism scale ($\alpha = $.89; Henry & Sears, 2002) to assess prejudice. Our measure of expressive threat was slightly adapted from the autonomy subscale of the basic psychological needs scale (Johnston & Finney, 2010): “I am free to decide for myself how to live my life,” “I feel pressured to think what others want me to think,” “I feel free to express my ideas and opinions,” “I feel like I can pretty much be myself,” “I feel free to be who I am,” “I can voice my opinion,” and “I feel controlled and pressured to have certain beliefs” ($\alpha = $ .92). Items were scored such that higher scores indicated more threat to expression.

They are easy to calculate in common statistical software you are likely using—like R, SPSS, or SAS. Make sure that if any of your items need to be reverse-scored you do so before calculating Cronbach's alpha.