Example 51.3 Ordinal Logistic Regression

Consider a study of the effects on taste of various cheese additives. Researchers tested four cheese additives and obtained 52 response ratings for each additive. Each response was measured on a scale of nine categories ranging from strong dislike (1) to excellent taste (9). The data, given in McCullagh and Nelder (1989, p. 175) in the form of a two-way frequency table of additive by rating, are saved in the data set Cheese by using the following program. The variable y contains the response rating. The variable Additive specifies the cheese additive (1, 2, 3, or 4). The variable freq gives the frequency with which each additive received each rating.

   data Cheese;
      do Additive = 1 to 4;
         do y = 1 to 9;
            input freq @@;
            output;
         end;
      end;
      label y='Taste Rating';
      datalines;
   0  0  1  7  8  8 19  8  1
   6  9 12 11  7  6  1  0  0
   1  1  6  8 23  7  5  1  0
   0  0  0  1  3  7 14 16 11
   ;

The response variable y is ordinally scaled. A cumulative logit model is used to investigate the effects of the cheese additives on taste. The following statements invoke PROC LOGISTIC to fit this model with y as the response variable and three indicator variables as explanatory variables, with the fourth additive as the reference level. With this parameterization, each Additive parameter compares an additive to the fourth additive. The COVB option displays the estimated covariance matrix. The ODDSRATIO statement computes odds ratios for all combinations of the Additive levels. The PLOTS(ONLY)= option produces a graphical display of the predicted probabilities and the odds ratios.

   ods graphics on;
   proc logistic data=Cheese plots(only)=(effect(polybar) oddsratio(range=clip));
      freq freq;
      class Additive (param=ref ref='4');
      model y=Additive / covb;
      oddsratio Additive;
      title 'Multiple Response Cheese Tasting Experiment';
   run;
   ods graphics off;

The "Response Profile" table in Output 51.3.1 shows that the strong dislike (y=1) end of the rating scale is associated with lower Ordered Values in the "Response Profile" table; hence the probability of disliking the additives is modeled.

The score chi-square for testing the proportional odds assumption is 17.287, which is not significant with respect to a chi-square distribution with 21 degrees of freedom . This indicates that the proportional odds assumption is reasonable. The positive value (1.6128) for the parameter estimate for Additive1 indicates a tendency toward the lower-numbered categories of the first cheese additive relative to the fourth. In other words, the fourth additive tastes better than the first additive. The second and third additives are both less favorable than the fourth additive. The relative magnitudes of these slope estimates imply the preference ordering: fourth, first, third, second.