Dear all,

I estimated a linear regression model on multiply imputed data. I regress income on a range of explanatory factors, amongst which a competence indicator. I assume that the effect of this competence differs between men and women so I included an interaction effect. My basic model syntax is

Code:
mi est, post: reg income i.sex##c.competences controls, vce(robust)
Now I assume that competences per se have a non-linear relationship with income (the difference of high and very high competences for income should not be as big). To examine that I estimated an interaction effect with the competence itself with:

Code:
mi est, post: reg income i.sex c.competences##c.competences controls, vce(robust)
And this interaction is significant, so it should have a non-linear effect.

I am now a bit unsure how to deal with this relationship. Thus far I have come across two different approaches

1.) Include this interaction along side my main interaction of interest in the model

Code:
mi est, post: reg income i.sex##c.competences c.competences##c.competences controls, vce(robust)
2.) Explicitly model the non-linear relationship. Here I came across the command
Code:
nl
but I am not sure what to do with my main interaction of interest (gender x competences) then

Does anybody have a good tipp here? I would be really grateful.

Cheery, Evelyn