Elaborations
For growth-curve modeling, another way to model nonlinearities is to use what is called a "piecewise growth model." For instance if there were five waves which were centered the linear time variable would be {-1,-2,0,1,2}. Say we want the slope to change after wave 3, we would add to the model two time variables: {-1,-2,0,0,0} and {0,0,0,1,2}. The first variable would model the linear slope for waves 1 to 3 and second for waves 3 to 5. Note these two slopes may vary.
