Suppose we would like to compare two raters using a kappa statistic but the
raters have different range of scores. This situation most often presents itself
where one of the raters did not use the same range of scores as the other rater.
Let us consider an example where two graduate students where asked to rate 12
movies based on a scale from 1-3. One rater used all of the three scores
possible while rating the movies whereas the other student did not like any of
the movies and therefore rated all of them as either a 1 or a 2. Thus, the range
of scores is the not the same for the two raters.
To obtain the kappa statistic in SAS we are going to use
proc freq with the test kappa statement. By default, SAS will only compute the kappa statistics if the two
variables have exactly the same categories, which is not the case in this
particular instance. We can get around this problem by adding a fake observation
and a weight variable shown below. The weight variable takes value of 1 for all
the real observations and value of 0 for the fake observation that we have just
added. The trick is then to use the option zeros for the statement.
data kappa; input rater1 rater2; datalines; 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 3 2 3 2 3 2 3 2 ; run; data kappa; if _n_=1 then do; rater1 =3; rater2 = 3; weight = 0; output; end; set kappa; weight = 1; output; run; proc freq data= kappa; tables rater1*rater2; test kappa; weight weight / zeros; run;
Statistics for Table of rater1 by rater2
Simple Kappa Coefficient
--------------------------------
Kappa 0.5000
ASE 0.1559
95% Lower Conf Limit 0.1944
95% Upper Conf Limit 0.8056
Test of H0: Kappa = 0
ASE under H0 0.1667
Z 3.0000
One-sided Pr > Z 0.0013
Two-sided Pr > |Z| 0.0027
Weighted Kappa Coefficient
--------------------------------
Weighted Kappa 0.5714
ASE 0.1323
95% Lower Conf Limit 0.3122
95% Upper Conf Limit 0.8307
Sample Size = 12