Cronbach's alpha - reliability coefficient

Cronbach's Alpha is a tool for assessing the reliability of scales (for example a quality of life instrument). Cronbach's alpha can be easily calculated from SAS Proc Corr.

To compute Cronbach's alpha for a set of variables, use the ALPHA option in PROC CORR as follows:
PROC CORR DATA=dataset ALPHA;
VAR item1-item10;
RUN;

SAS website provides an example about calculating the Cronbach's alpha.
Very often, 95% confidence interval may be required, the calculation is not straightforward, but there are SAS macros available from the SAS web site.

Some references about Cronbach's alpha can be found below:

• http://en.wikipedia.org/wiki/Cronbach
• http://support.sas.com/documentation/cdl/en/procstat/59629/HTML/default/procstat_corr_sect019.htm
• http://www.creative-wisdom.com/teaching/assessment/alpha.html
• http://www2.sas.com/proceedings/sugi26/p246-26.pdf

To assess the reliability of an instrument, the good reliability features include:

• Internal consistency = Cronbach's alpha >= 0.70 for new measures
• Stability = reliability coefficient >= 0.70
• Equivalence = Kappa statistic >= 0.61

Reference: Nunnally & Bernstein, 1994; Landis & Koch, 1977

In one of comments on FDA's guidance on PROM (patient reported outcome measures), Cronbach's alpha was cited to measure Internal Consistency and construct validity (with scale analysis) - Cronbach's alpha > 0.70
http://www.fda.gov/ohrms/dockets/dockets/06d0044/06d-0044-EC13-Attach-1.pdf