Interactions involving Categorical Predictors
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1 Interactions involving Categorical Predictors Today s Class: To CLASS or not to CLASS: Manual vs. program-created differences among groups Interactions of continuous and categorical predictors Interactions among categorical predictors CLP 944: Lecture 2c 1
2 Categorical Predictors (3+ Groups) Two alternatives for how to include grouping predictors 1. Manually create and include dummy-coded group contrasts Need g 1 contrasts for g groups, added all at once, treated as continuous (WITH in SPSS, by default in SAS, c. in STATA) Corresponds more directly to linear model representation Can be easier to set own reference group and contrasts of interest 2. Let the program create and include group contrasts for you Treated as categorical: BY in SPSS, CLASS in SAS, i. in STATA SPSS and SAS: reference = highest/last group; STATA: reference = lowest/first group Can be more convenient if you have many groups, want many contrasts, or have interactions among categorical predictors Program marginalizes over main effects when estimating other effects CLP 944: Lecture 2c 2
3 Categorical Predictors Treated as Continuous Model: Treatgroup variable: Control=0, Treat1=1, Treat2=2, Treat3=3 New variables d1= 0, 1, 0, 0 difference between Control and T1 to be created d2= 0, 0, 1, 0 difference between Control and T2 for the model: d3= 0, 0, 0, 1 difference between Control and T3 How does the model give us all possible group differences? By determining each group s mean, and then the difference Control (Reference) Treatment 1 Treatment 2 Treatment The model for the 4 groups directly provides 3 differences (control vs. each treatment), and indirectly provides another 3 differences (differences between treatments) CLP 944: Lecture 2c 3
4 Categorical Predictors Treated as Continuous Model: Control (Reference) Treatment 1 Treatment 2 Treatment Alt Group Ref Group Difference Control vs. T1 = Control vs. T2 = Control vs. T3 = T1 vs. T2 = T1 vs. T3 = T2 vs. T3 = CLP 944: Lecture 2c 4
5 Main effects with manual dummy codes Alt Group Ref Group Difference Control vs. T1 = β β β β Control vs. T2 = β β β β Control vs. T3 = β β β β Note the order of the equations: the reference group mean is subtracted from the alternative group mean. T1 vs. T2 = β β β β β β T1 vs. T3 = β β β β β β T2 vs. T3 = β β β β β β TITLE "Manual Contrasts for 4-Group Diffs"; PROC MIXED DATA=dataname METHOD=REML; MODEL y = d1 d2 d3 / SOLUTION; CONTRAST "Omnibus df=3 main effect F-test" d1 1, d2 1, d3 1; ESTIMATE "Control " intercept 1 d1 0 d2 0 d3 0; ESTIMATE "T1 " intercept 1 d1 1 d2 0 d3 0; ESTIMATE "T2 " intercept 1 d1 0 d2 1 d3 0; ESTIMATE "T3 " intercept 1 d1 0 d2 0 d3 1; ESTIMATE "Control vs. T1" d1 1 d2 0 d3 0; ESTIMATE "Control vs. T2" d1 0 d2 1 d3 0; ESTIMATE "Control vs. T3" d1 0 d2 0 d3 1; ESTIMATE "T1 vs. T2" d1-1 d2 1 d3 0; ESTIMATE "T1 vs. T3" d1-1 d2 0 d3 1; ESTIMATE "T2 vs. T3" d1 0 d2-1 d3 1; RUN; In SAS ESTIMATE statements (or SPSS TEST or STATA LINCOM), the variables refer to their betas; the numbers refer to the operations of their betas. Intercepts are used only in predicted values. Positive values indicate addition; negative values indicate subtraction. CLP 944: Lecture 2c 5
6 Interactions with manual dummy codes When using manual dummy-codes for group contrasts treated as continuous, any interactions have to be specified with each contrast For example, adding an interaction of treatgroup with age (0=85): y β β d1 β d2 β d3 β Age 85 β d1 Age 85 β d2 Age 85 β d3 Age 85 e TITLE "Group by Age for 4-Group Variable Treated as Continuous"; PROC MIXED DATA=dataname METHOD=REML; MODEL y = d1 d2 d3 age d1*age d2*age d3*age / SOLUTION; CONTRAST "Omnibus df=3 SIMPLE effect F-test" d1 1, d2 1, d3 1; CONTRAST "Omnibus df=3 interaction F-test" d1*age 1, d2*age 1, d3*age 1; ESTIMATE "Age Slope for Control" age 1 d1*age 0 d2*age 0 d3*age 0; ESTIMATE "Age Slope for T1" age 1 d1*age 1 d2*age 0 d3*age 0; ESTIMATE "Age Slope for T2" age 1 d1*age 0 d2*age 1 d3*age 0; ESTIMATE "Age Slope for T3" age 1 d1*age 0 d2*age 0 d3*age 1; ESTIMATE "Age Slope: Control vs. T1" d1*age 1 d2*age 0 d3*age 0; ESTIMATE "Age Slope: Control vs. T2" d1*age 0 d2*age 1 d3*age 0; ESTIMATE "Age Slope: Control vs. T3" ESTIMATE "Age Slope: T1 vs. T2" d1*age 0 d2*age 0 d3*age 1; d1*age -1 d2*age 1 d3*age 0; ESTIMATE "Age Slope: T1 vs. T3" d1*age -1 d2*age 0 d3*age 1; ESTIMATE "Age Slope: T2 vs. T3" d1*age 0 d2*age -1 d3*age 1; CLP 944: Lecture 2c 6
7 Using BY/CLASS/i. statements instead Designate as categorical in program syntax If you let SAS/SPSS do the dummy coding via CLASS/BY, then the highest/last group is default reference In SAS 9.4 you can change reference group: REF= level FIRST LAST but it changes that group to be last in the data ( confusing) Type III test of fixed effects provide omnibus tests by default LSMEANS/EMMEANS can be used to get all means and comparisons without specifying each individual contrast If you let STATA do the dummy coding via i.group, then the lowest/first group is default reference Can change reference group, e.g., last = ref ib(last).group CONTRAST used to get omnibus tests (not provided by default) MARGINS can be used to get all means and comparisons with much less code than describing each individual contrast CLP 944: Lecture 2c 7
8 Main effects of Categorical Predictors TITLE "Program-Created Contrasts for 4-Group Diffs via CLASS"; PROC MIXED DATA=dataname ITDETAILS METHOD=REML; CLASS treatgroup; MODEL y = treatgroup / SOLUTION; LSMEANS treatgroup / DIFF=ALL; The LSMEANS line above gives you all of the following note that one value has to be given for each possible level of the categorical predictor in order of data ESTIMATE "Control " intercept 1 treatgroup ; ESTIMATE "T1 " intercept 1 treatgroup ; ESTIMATE "T2 " intercept 1 treatgroup ; ESTIMATE "T3 " intercept 1 treatgroup ; ESTIMATE "Control vs. T1" treatgroup ; ESTIMATE "Control vs. T2" treatgroup ; ESTIMATE "Control vs. T3" treatgroup ; ESTIMATE "T1 vs. T2" treatgroup ; ESTIMATE "T1 vs. T3" treatgroup ; ESTIMATE "T2 vs. T3" treatgroup ; CONTRAST "Omnibus df=3 main effect F-test" treatgroup , treatgroup , CLASS gives this contrast by default treatgroup ; CLASS statement means make my dummy codes for me When predicting intercepts, 1 means for that group only When predicting group differences, contrasts must sum to 0; here 1 = ref, 1 = alt, and 0 = ignore Can also make up whatever contrasts you feel like using DIVISOR option: ESTIMATE " of Treat groups" intercept 1 treatgroup / DIVISOR=3; ESTIMATE "Control vs. of Treat groups" treatgroup / DIVISOR=3; RUN; CLP 944: Lecture 2c 8
9 Interactions with Categorical Predictors For example, adding an interaction of treatgroup with age (0=85): TITLE "Group by Age for 4-Group Variable Modeled as Categorical"; PROC MIXED DATA=dataname METHOD=REML; CLASS treatgroup; MODEL y = treatgroup age treatgroup*age / SOLUTION; * To explain interaction as how group diffs depend on age: LSMEANS treatgroup / DIFF=ALL AT (age)=(-5); * group intercept diffs at age 80; LSMEANS treatgroup / DIFF=ALL AT (age)=(0); * group intercept diffs at age 85; LSMEANS treatgroup / DIFF=ALL AT (age)=(5); * group intercept diffs at age 90; * To explain interaction as how age slope depends on group: ESTIMATE "Age Slope for Control" age 1 treatgroup*age ; ESTIMATE "Age Slope for T1" age 1 treatgroup*age ; ESTIMATE "Age Slope for T2" age 1 treatgroup*age ; ESTIMATE "Age Slope for T3" age 1 treatgroup*age ; ESTIMATE "Age Slope: Control vs. T1" treatgroup*age ; ESTIMATE "Age Slope: Control vs. T2" treatgroup*age ; ESTIMATE "Age Slope: Control vs. T3" treatgroup*age ; ESTIMATE "Age Slope: T1 vs. T2" treatgroup*age ; ESTIMATE "Age Slope: T1 vs. T3" treatgroup*age ; ESTIMATE "Age Slope: T2 vs. T3" treatgroup*age ; Can also make up whatever contrasts you feel like using DIVISOR option: ESTIMATE " Age Slope in Treat groups" age 1 treatgroup*age / DIVISOR=3; ESTIMATE "Age Slope: Control vs. of Treat" treatgroup*age / DIVISOR=3; RUN; CLP 944: Lecture 2c 9
10 Categorical Predictors = Marginal Effects Letting the program build contrasts for categorical predictors (instead of creating manual dummy codes) does the following: Allows LSMEANS/EMMEANS/MARGINS (for cell means and differences) Provides omnibus (multiple df) multivariate Wald tests for group effects Marginalizes the group effect across interacting predictors omnibus F-tests represent marginal main effects (instead of simple) e.g., MODEL y = Treatgroup sexmw Treatgroup*sexMW (in which Treatgroup is always categorical ) Type 3 Tests of Fixed Effects Interpretation if sexmw is continuous (no CLASS) Interpretation if sexmw is categorical on CLASS sexmw Marginal diff across groups Marginal diff across groups Treatgroup Group diff if sexmw=0 Marginal diff across sexes Treatgroup*sexMW Interaction Interaction CLP 944: Lecture 2c 10
11 Interactions Among Categorical Predictors By default (i.e., as in ANOVA ): Model includes all possible higher-order interactions among categorical predictors Software does this for you; nonsignificant interactions usually still are kept in the model (but only significant interactions are interpreted) This is very different from typical practice in regression! Omnibus marginal main effects are provided by default i.e., what we ask for via CONTRAST using manual group contrasts But are basically useless if given significant interactions Omnibus interaction effects are provided i.e., what we ask for via CONTRAST using manual group contrasts But are basically useless in actually understanding the interaction Let s see how to make software give us more useful info CLP 944: Lecture 2c 11
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