# Thread: Reporting Multiple Regression with Categorical Variable(s)

1. ## Reporting Multiple Regression with Categorical Variable(s)

Hi everyone,

I'm overthinking one of my analyses and need a community sanity check.

For simplicity's sake, let's say that my basic multiple regression model I am using for the first iteration of my analysis currently contains the following variables:

Step 1: Centered continuous variable (IV 1)
Step 2: Categorical variable (conditions coded 1, 2, and 3), (IV 2)
Step 3: Interaction between IV 1 and IV 2

In a second iteration of this model, IV 2 would be replaced by two dummy codes and then basically the same analyses would be conducted with those dummy codes. So, it would be:

Step 1: Centered continuous variable (IV 1)
Step 2: Two dummy codes in place of original categorical variable (Dummy1, Dummy2)
Step 3: Interactions between IV 1 and each of the dummy codes

I am being asked to analyze and report both iterations of this model.

My question is: What is the purpose/utility of reporting the first iteration of this analysis, and how does that data inform how you proceed from there?

Does iteration #1 provide main effects information for the categorical variable, while iteration #2 provides information on the contrasts between conditions? If so, what happens when the "main effect" is not significant but Dummy1 is (not Dummy2)? Do I not report the dummy code results if the "overall" categorical variable from the first iteration is not significant--even though the effects of the dummy codes (one sig, one not sig) may be "cancelling each other out"? Or am I not thinking about this correctly?

Thanks!

tbf

2. ## Re: Reporting Multiple Regression with Categorical Variable(s)

Hi, I don't understand what " IV 2 would be replaced by two dummy codes" means. Since IV 2 is already a categorical variable with 3 categories, these categories would be already represented as a dummy variable in the context of a regression analysis, represented by 3 numbers. So how exactly is the coding in the second model for this variable?

3. ## Re: Reporting Multiple Regression with Categorical Variable(s)

Hi mmercker,

In the first iteration of this analysis, the categorical variable contains the values 1, 2, and 3 (random numbers associated with experimental conditions 1, 2, and 3 that participants had been randomly assigned to for the study).

In the second iteration of this analysis, I created two dummy codes so that I could directly examine comparisons between my experimental conditions and my control condition. Conditions 1 and 2 are my experimental conditions, and condition 3 is my control condition. Thus, I created Dummy1 (comparing condition 1--recoded as 1--with my control condition, coded as 0). Then I created Dummy2 (comparing condition 2--recoded as 1--with my control condition, coded as 0).

Does that make more sense? I can provide more details if needed.

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