Dear. Statalist

I am trying to do a piecewise regression with panel data but I can't find out much about this topic. Anyone have suggestions on what STATA command to use? Also what are the issues with doing a piecewise regression with paneldata?

This is my equation: nl (KOSTBHG = SIZE*{b1} + (SIZE>{c})*( SIZE-{c})*{b2}), variables(SIZE) initial(b1 0 c 28 b2 0) noconstant, if(Year==2015)