Linear Regression Reference Group
Dear JASP experts,
I am running a linear regression model where an independent variable (categorical) has three groups (e.g., 0, 1, 2). Could you please let me know which group will be the reference group among three?
Best,
JW
Comments
Hi JW,
It depends on how you set this up. If you do a linear regression with a categorical variable with values 0, 1, 2, then it's treated as a continuous covariate. If you'd like to test whether a categorical variable is relevant, then we recommend doing an ANOVA/ANCOVA, as you don't have to worry about dummy coding. The test then checks whether the grouping variable (thus, whether the labels 0, 1, 2) is necessary or not. The overall group effect is absorbed in the intercepts. In general, when a reference group is needed the first level is taken as reference group, which you can find by ticking the variable in the data viewer.
Cheers,
Alexander