Saturday, February 14, 2009

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:
VAR item1-item10;

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:

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

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