Dear All,
I run a model in which a have one variable (Var1) interacting with three other variables (Var2, Var3 and Var4).
Var1 has three categories and Var2, Var3 and Var4 are binary variable indicating the presence or absence of symptoms.
Is it possible to suggest a text showing the best way to interpret interactions of one variable with two or more variables?
Thanks a lot.
Regards,
Rather than suggest a text, I suggest that you examine and try to understand an interaction plot. Here is an example (scroll down): https://documentation.sas.com/doc/en/statug/15.2/statug_glm_examples03.htm
Interactions exist when the colored lines are not parallel to each other. Note that the red line (disease 2) is highest for drugs 1 and 2 and lowest for drugs 3 and 4. That is an interaction between drugs and diseases. Had the red line not been present and drug 4 removed so only three drugs and 2 diseases, the green and blue lines are (approximately) parallel to each other, drugs 1 and 3 (approximately) do not interact with disease. The difference is the main effect of drug.
How much “non-parallel-ness” is allowed? That would be determined by the actual F-test for the interaction.
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