topic Re: Interpreting Interaction Effect in Proc Genmod (Categorical vs. Categorical with multiple levels in Statistical Procedures

Interpreting Interaction Effect in Proc Genmod (Categorical vs. Categorical with multiple levels)

varatt90 — Thu, 07 Jul 2022 19:10:31 GMT

Hi all,

I was wondering if someone can help me interpret the interaction variables presented in the output below.

If we pretend that the interaction term in the Type 3 table is significant, my understanding is that:

- interaction between being a boy (vs. Girl) and being Depressed (vs. Not depressed) - significant

- interaction between being other (vs. Girl) and being depressed (vs. not depressed) - significant

The values for girl is not provided as it is the reference group that I have chosen.

Is this correct? Any help would be appreciated!

Re: Interpreting Interaction Effect in Proc Genmod (Categorical vs. Categorical with multiple levels

StatDave — Fri, 08 Jul 2022 19:10:24 GMT

When there is interaction in the model, you interpret the effect of each variable at each level of the other variable it interacts with. To do that, use the SLICE statement. For example:

slice sex*depression/ sliceby=depression;

slice sex*depression/ sliceby=sex;

Re: Interpreting Interaction Effect in Proc Genmod (Categorical vs. Categorical with multiple levels

PaigeMiller — Fri, 08 Jul 2022 20:22:32 GMT

In addition to the good advice from @StatDave you should also take advantage of the excellent plots available in PROC GENMOD via the EFFECTPLOT statement to help you interpret the interaction. There are interaction plots, SLICEFIT plots, and SLICEBY plots.

Re: Interpreting Interaction Effect in Proc Genmod (Categorical vs. Categorical with multiple levels

varatt90 — Mon, 11 Jul 2022 19:18:40 GMT

Here, I used spliceby = sex

So from the Analysis of GEE Parameter Estimates Tables, I can identify the significance of whether or not interaction exists. And I can use this Simple Differences Table to identify significance between sex groups that have depression. Is that correct?

Re: Interpreting Interaction Effect in Proc Genmod (Categorical vs. Categorical with multiple levels

StatDave — Mon, 11 Jul 2022 19:32:35 GMT

That appears to be the results of a statement like
slice sex*depression / sliceby=depression diff;
But, yes, that table lets you compare the sex levels when depression=yes.

Re: Interpreting Interaction Effect in Proc Genmod (Categorical vs. Categorical with multiple levels

varatt90 — Mon, 11 Jul 2022 19:34:23 GMT

Yes! My apologies! I did: slice sex*depression / sliceby(depression) diff oddsratio CL ;

Re: Interpreting Interaction Effect in Proc Genmod (Categorical vs. Categorical with multiple levels

varatt90 — Mon, 11 Jul 2022 19:49:40 GMT

When I specify "effectplot interaction (x=depression)" where Y = smoking status into my code, this is what I see. Is this what you meant?

Re: Interpreting Interaction Effect in Proc Genmod (Categorical vs. Categorical with multiple levels

StatDave — Mon, 11 Jul 2022 20:41:00 GMT

Yes, that provides a graphical depiction of the comparison results from your SLICE statement.

Re: Interpreting Interaction Effect in Proc Genmod (Categorical vs. Categorical with multiple levels

SteveDenham — Wed, 13 Jul 2022 12:20:30 GMT

Yes, those plots look exactly like the ones I used to illustrate an interaction years ago when I taught an introductory stats class.

SteveDenham

Re: Interpreting Interaction Effect in Proc Genmod (Categorical vs. Categorical with multiple levels

PaigeMiller — Wed, 13 Jul 2022 12:33:29 GMT

@varatt90 wrote:

When I specify "effectplot interaction (x=depression)" where Y = smoking status into my code, this is what I see. Is this what you meant?

Yes, these are the plots. I find looking at them helpful in some ways, girl seems to have opposite sign on the slope to boys and opposite sign on the slope to other. That's a useful piece of information that isn't immediately obvious looking at the tables of output. Another point is that the slope of other is flatter than the slope of boys and flatter than the slope of girls; again this is a piece of information that isn't immediately obvious looking at the tables of output. Finally, girls shows a bigger range from "No" to "Yes" than boys or other on both plots.

Re: Interpreting Interaction Effect in Proc Genmod (Categorical vs. Categorical with multiple levels

varatt90 — Wed, 13 Jul 2022 13:57:16 GMT

Thank you for interpreting that graphs! It was nice to see what to look out for when reading the graphs.

Re: Interpreting Interaction Effect in Proc Genmod (Categorical vs. Categorical with multiple levels

varatt90 — Wed, 13 Jul 2022 14:19:52 GMT

You mentioned "When there is interaction in the model, you interpret the effect of each variable at each level of the other variable it interacts with. To do that, use the SLICE statement. "

Just to confirm, if we didn't assume the interaction effect in the Type 3 table was significant and went by actually what we see in the output (i.e., the interaction term in the Type 3 table was not significant.), then I would not be able to interpret the effect of each variable at each level of the other variable it interacts with. Is this correct?

Re: Interpreting Interaction Effect in Proc Genmod (Categorical vs. Categorical with multiple levels

StatDave — Wed, 13 Jul 2022 14:26:33 GMT

If there is no interaction then that means that the effect of each variable is the same at all levels of the other variable. So, that greatly simplifies the situation. Each variable just has an overall effect.

Re: Interpreting Interaction Effect in Proc Genmod (Categorical vs. Categorical with multiple levels

varatt90 — Wed, 13 Jul 2022 14:28:36 GMT

Great! Thank you so much for clarifying that!

Re: Interpreting Interaction Effect in Proc Genmod (Categorical vs. Categorical with multiple levels

varatt90 — Wed, 13 Jul 2022 14:33:12 GMT

Are there any resources that you think would be helpful reading regarding interactions between categorical variables (esp when the outcome variable is also categorical)?

Re: Interpreting Interaction Effect in Proc Genmod (Categorical vs. Categorical with multiple levels

StatDave — Wed, 13 Jul 2022 14:37:06 GMT

See this note that discusses models with interaction in general and testing hypotheses in such models. In particular, Example 3 illustrates and discusses an interaction model in a binary response model.

Re: Interpreting Interaction Effect in Proc Genmod (Categorical vs. Categorical with multiple levels

PaigeMiller — Wed, 13 Jul 2022 14:38:13 GMT

@varatt90 wrote:
Thank you for interpreting that graphs! It was nice to see what to look out for when reading the graphs.

Some famous person once created the hashtag #APictureIsWorthAThousandNumbers

Re: Interpreting Interaction Effect in Proc Genmod (Categorical vs. Categorical with multiple levels

varatt90 — Wed, 13 Jul 2022 14:38:28 GMT

Thank you so much!!!

Re: Interpreting Interaction Effect in Proc Genmod (Categorical vs. Categorical with multiple levels

varatt90 — Wed, 13 Jul 2022 14:41:13 GMT

Haha definitely a wise person...who also probably loved interaction plots.