Monday, August 20, 2018

AC1 Coefficient implemented in the FREQ Procedure of SAS

As of SAS/STAT version 14.2, the AC1 (see Gwet, 2008) and PABAK (see Byrt, Bishop, and Carlin, 1993) agreement coefficients can be calculated using the FREQ procedure of SAS, in addition to Cohen's Kappa. Therefore, SAS users no longer need to use another software to obtain theses statistics.

SAS users should nevertheless be aware that by default the FREQ procedure systematically deletes all observations with one missing value.  Consequently, the results obtained with SAS may differ from those obtained with other r functions available in several packages, if your dataset contains missing ratings.  An option is available for instructing the FREQ procedure to treat missing values as true categories. However, this option is useless for the analysis of agreement among raters.  What would be of interest is for Proc FREQ developers to allow for the marginals associated with rater1 and rater2 to be calculated independently.  That is, if a rating is available from rater1 then it should be used for calculating rater1's marginals whether it is available from rater2 or not.

One last comment.  The coefficient often referred to by researchers as PABAK is also known (perhaps more rightfully so) as the Brennan-Prediger coefficient.  It was formally studied by Brennan & Prediger (1981), 13 years earlier.


Byrt, T., Bishop, J., and Carlin, J. B. (1993). Bias, prevalence and Kappa. Journal of Clinical Epidemiology, 46, 423-429.

Brennan, R. L., and Prediger, D. J. (1981). Coefficient Kappa: some uses, misuses, and alternatives. Educational and Psychological Measurement, 41, 687-699. 

Gwet, K. L. (2008). Computing inter-rater reliability and its variance in the presence of high agreement. British Journal of Mathematical and Statistical Psychology, 61, 29-48.