A note on Cronbach's alpha
[http://www.bmj.com/cgi/content/full/314/7080/572 Bland JM, Altman DG (1997) Statistics notes: Cronbach's alpha. BMJ 314 572]
The above article suggests rules of thumb for Cronbach's $$\alpha$$ and examples of its use.
Cronbach's alpha is defined as
$$\frac{k}{k-1} (1 - \frac{1}{\mbox{Variance of total scores}} \mbox{Sum of item variances})$$