Dear all,
i hope you are doing good

i'm working on the relationship between financial constraints ( using SA index) and R&D investments applying a dynamic panel threshold model.

among the control variables, i would like to use the firm size but the problem here is that SA index = (-0.737*Size)+(0.043*Size*Size)- (0.040*Age). here the independent variable ( SA index) contains a control variable. is it normal ? or should i remove size a control variable from the regression even i'm using a dynamic threshold model ??

Best regards
SEDKI