The CATMOD Procedure

In an example from Ries and Smith (1963), the choice of detergent brand (Brand = M or X) is related to three other categorical variables: the softness of the laundry water (Softness = soft, medium, or hard), the temperature of the water (Temperature = high or low), and whether the subject was a previous user of Brand M (Previous = yes or no). The linear response function, which could also be specified as RESPONSE MARGINALS , yields one probability, Pr(brand preference=M), as the response function to be analyzed. Two models are fit in this example: the first model is a saturated one, containing all of the main effects and interactions, while the second is a reduced model containing only the main effects. The following statements produce Output 32.1.1 through Output 32.1.4:

data detergent;
   input Softness $ Brand $ Previous $ Temperature $ Count @@;
   datalines;
soft X yes high 19   soft X yes low 57
soft X no  high 29   soft X no  low 63 
soft M yes high 29   soft M yes low 49
soft M no  high 27   soft M no  low 53
med  X yes high 23   med  X yes low 47
med  X no  high 33   med  X no  low 66
med  M yes high 47   med  M yes low 55
med  M no  high 23   med  M no  low 50
hard X yes high 24   hard X yes low 37
hard X no  high 42   hard X no  low 68
hard M yes high 43   hard M yes low 52
hard M no  high 30   hard M no  low 42
;

title 'Detergent Preference Study';
proc catmod data=detergent;
   response 1 0;
   weight Count;
   model Brand=Softness|Previous|Temperature / freq prob;
   title2 'Saturated Model';
run;

The "Data Summary" table (Output 32.1.1) indicates that you have two response levels and twelve populations.

The "Population Profiles" table in Output 32.1.2 displays the ordering of independent variable levels as used in the table of parameter estimates.

Since Brand M is the first level in the "Response Profiles" table (Output 32.1.3), the RESPONSE statement causes Pr(Brand=M) to be the single response function modeled.

The "Analysis of Variance" table in Output 32.1.4 shows that all of the interactions are nonsignificant.

Therefore, a main-effects model is fit with the following statements:

   model Brand=Softness Previous Temperature
       / clparm noprofile design;
   title2 'Main-Effects Model';
run;
quit;

The PROC CATMOD statement is not required due to the interactive capability of the CATMOD procedure. The NOPROFILE option suppresses the redisplay of the "Response Profiles" table. The CLPARM option produces 95% confidence limits for the parameter estimates. Output 32.1.5 through Output 32.1.7 are produced.

The design matrix in Output 32.1.5 displays the results of the differential-effects modeling used in PROC CATMOD.

The analysis of variance table in Output 32.1.6 shows that previous use of Brand M, together with the temperature of the laundry water, is a significant factor in whether a subject prefers Brand M laundry detergent. The table also shows that the additive model fits since the goodness-of-fit statistic (the residual chi-square) is nonsignificant.

The chi-square test in Output 32.1.7 shows that the Softness parameters are not significantly different from zero; as expected, the Wald confidence limits for these two estimates contain zero. So softness of the water is not a factor in choosing Brand M.

The negative coefficient for Previous (–0.0711) indicates that the first level of Previous (which is shown to be 'no') is associated with a smaller probability of preferring Brand M than the second level of Previous (with coefficient constrained to be 0.0711 since the parameter estimates for a given effect must sum to zero). In other words, previous users of Brand M are much more likely to prefer it than those who have never used it before.

Similarly, the positive coefficient for Temperature indicates that the first level of Temperature (which, from the "Population Profiles" table, is 'high') has a larger probability of preferring Brand M than the second level of Temperature. In other words, those who do their laundry in hot water are more likely to prefer Brand M than those who do their laundry in cold water.