I am trying to creat a multiple regression model with a forward stepwise procedure. Predictors are air temperature, soil temperature, PAR and snow depth. I also want to see if there are some interaction effect between snow depth and other factors that can also be included in the model. Should I include the interaction as factors (e.g. Ta:Snow depth) when going through the stepwise procedure (main effect may not be included) or make another model to illustrate the interaction effect separately (include both main and interaction effect)?
PS:I am using R to do this.
Thanks a lot in advance!