Hi. I running a path model to examine the relationship between x and y while including covariates and mediators. I would like to account for a non-linear relationship between x and y, as indicated when I examine the relationship using ordinary regression. Do you have any recommendations on how to best account for a non-linear relationship? I have tried to include X^2 in the model, having it modeled just as X is and allowing the two to covary. However the model is non-positive definite when I do this.
I have also considered including an additional term for piecewise regression. Would it follow the normal X term and do they covary?
Any advice that you could provide would be very helpful!