I have the equation $$ y = \beta_1 + \beta_2 x_2 + \beta_3 x_3 + \beta_4 x_4 + \beta_5 x_5 + \beta_6 x_6 + \beta_7 x_7 + \beta_8 x_8 + \beta_9 x_9 + \varepsilon $$
The dependent variable is weight of child at birth, and independent variables are Age, BMI, Social Class and different types of Vitamin intake.
Clearly there are 2 categorical variables (bmi and social) which I have used SPSS to create dummy variables for. I therefore now have 7 dummy variables (By leaving one out for each).
Must I now include interactions? Im not sure what this means and how this would be incorporated in the model. Do i multiply each dummy variable by every other dummy variable in the model?