I'm doing a linear mixed model using lme.
In my adjustement factors, I have a continuous varaible (named X1). And I want to check the linearity of this variable using a spline function or a fractional polynomial but I don't succeed.
Can someone help me ? How to do that ?
Ex with spline :
lme(Y ~ T + X + pspline(X1, 4), data = df, random = ~ T | SUBJECT )
But then, I don't know how to conclude for the linearity.