Base category not displayed

[Ethan Gaala]

Fellow stata users, I am having trouble getting state to display the base category of the interaction between two variables that are both three-item categorical variables. Ideally, I should be seeing 9 points of interaction but I'm only seeing 6. And even then the base category is not displayed. When I output the results in an estout table, the base category is shown but not the base I was expecting. See below:

* Example generated by -dataex-. For more info, type help dataex
clear
input byte(Accountability_over_Speed Approval_of_Protest Religiousity_3)
 2  0  2
 2  1  2
 0  0  2
 2  1  0
.b  0  1
 0  0  2
 2  1  2
 1  0  1
 2  0  2
 0  0  2
 2  1  2
 2 .b  2
 2  1  2
 0  0  2
 2  0  2
 0  0  2
 0  1  2
 0  1  1
 0  0  2
 0  0  2
 0  1  2
 0  0  2
.b  0  2
 0  1  1
 2  1  2
 2  1  2
 2  0  2
 2  0  0
 0  0  2
 0  0  2
 0  0  0
 2  1  2
 2  0  2
 2 .b  1
 0  1  1
 2  0 .a
 2  0  2
 0  0  2
 0  0  1
 0  1  1
 2  0  1
 2  1  2
 0  1  2
 2  0  1
 2  1  1
 2  0  2
 0  1  2
 0  0  1
 2  1  1
 2  0  2
 2 .b  1
 0  0  2
 0  0  1
 0  0  1
.b  0  1
 0  1  1
 0  0  2
 0  0  2
 2  0  1
 0  0  2
 2  0  1
 0  1  2
 0  1  1
.b  0  1
 2  1  2
 1  0  2
 2  0  1
 0  0  2
 0  1  0
 2  1  1
 0 .b .b
 0  0  2
 0  0  2
 0  0  1
 2  0  1
 2  1  2
 2  0  2
 0  1  2
.b  0  2
 2  1  2
 0  1  1
 0  0  2
 1 .b .b
 0  0  1
 2  0  1
 2  1  2
 0  0  0
 0  0  1
.b .b  2
 2  1  2
 2  0  1
 0  1  2
 0  0  2
 2  1  2
 2  0  1
 0  0  2
 2  0  2
 0  0  2
.b  0  2
 0  1  2
end
label values Accountability_over_Speed Accountability_over_Speed
label def Accountability_over_Speed 0 "Speed", modify
label def Accountability_over_Speed 1 "Unsure", modify
label def Accountability_over_Speed 2 "Accountability", modify
label values Approval_of_Protest Approval_of_Protest
label def Approval_of_Protest 0 "No", modify
label def Approval_of_Protest 1 "Yes", modify
label values Religiousity_3 Religiousity_3
label def Religiousity_3 0 "Not Religious", modify
label def Religiousity_3 1 "Somewhat Religious", modify
label def Religiousity_3 2 "Highly Religious", modify

logit Approval_of_Protest i.Accountability_over_Speed##i.Religiousity_3, base
Results attached

[Clyde Schechter]

Ideally, I should be seeing 9 points of interaction but I'm only seeing 6.

Neither part of that sentence is correct. Second part first: your output clearly shows 4 categories for the interaction, not 6.

And that is exactly what you should be seeing. When you have an interaction between two categorical variables, having m and n levels, the number of interaction levels will be (m-1) x (n-1). And, indeed, with m = n = 3, you should be seeing (3-1)x(3-1) = 2x2 = 4, which is precisely what you have.

There is nothing wrong here except your expectations.

If you want to see the predicted outcome probabilities in each of the 9 combinations of accountability and religiosity, you can follow the logistic regression with:

margins i.Accountability_over_Speed##i.Religiousity_3

and you will get that.

[Ethan Gaala]

Thank you, Clyde Schechter. This is helpful!