# Two treatments - How do I compute reliability estimates like Cronbach's $\alpha$?

I randomized my survey, and now I have my data on SPSS (and all cleaned up) but when trying to run my Cronbach's $\alpha$ I have a problem.

Because I have two treatments (sexualised and normal) all my measures are present twice in my data set. For example, “similarity” has 3 items but now I have 6 variables: 3 for the sexualized and 3 for the normal condition.

Sim1
Sim2
Sim3
aSim1
aSim2
aSim3


SPSS will not allow me to combine 2 treatments because I have no data that has answers for the 6 (obviously).

Do I calculate them separately and then get the average between the two, or do I report both Cronbach's $\alpha$s for the 2 treatments?

• What do you want to do with $\alpha$ and how do you plan to use these data afterwards?
– Gala
Jul 18, 2013 at 20:39
• Also what's your sample size?
– Gala
Jul 18, 2013 at 20:59
• Can you present examples of items? The likely answer is that you calculate the alpha separately. Jul 18, 2013 at 21:09
• @Behacad +1 to your comment. In fact you definitely cannot meaningfully do anything else that calculating alpha separately on each sub-sample in my opinion but the likely answer is that you shouldn't be calculating alpha at all.
– Gala
Jul 18, 2013 at 21:16
• Ok for example I am using purchase intention (DV) made up of 3 measures: one being perceived social norm. To make sure I am using valid and reliable measures I need to group all my items of social norm to get the cronbachs. but Because I have 2 treatments, sexualized and normal it doesn't allow me to combine the items for social norm. I have 3 items for social norm but because of the 2 treatmens it has multiplied my social norm items by 2. Thus to present a cronbachs alpha should I do them separately and then get the average? or report the 2 separately? thanks!! Jul 18, 2013 at 21:54