Piecewise linear growth-curve modeling using Proc Mixed

[kleger]

Hello,

I am trying to model blood pressure growth trajectories  across two distinct periods (reactivity and recovery from a stressor). I am looking to examine rates of linear growth for levels of bp for each person within each period. I also include person-level predictors.

I am currently modeling blood pressure as a function of each time point. However, there are 8 time points (4 during the reactivity phase and 4 during the recovery phase), and I would like to divide time into two periods, before minute 4 and after minute 4.

Here is my SAS code modeling each time point (I use SAS 9.4):

proc mixed data=third;
class condition time;
model systolic =  time|condition;
repeated time / sub=id type=un;
lsmeans time|condition;
run; 

How can I create a spline model where I can get two separate coefficients (one before time=4 and one after time=4)?

Thank you!

[WarrenKuhfeld]

Change my time of 12 to your time of 4.

data x;
   set sashelp.class(rename=(age=time));
   time1 = (time <= 12) * time;
   time2 = (time >  12) * time;
   run;

[Rick_SAS]

You mentioned splines, but your code does not appear to use splines. When you use the EFFECT statement to construct the spline, put a knot at time=4. That will enable you to create a piecewise linear model. See the second example in the article "Nonsmooth models and spline effects."

[kleger]

Thank you, Rick! This was very helpful.