how to convert a continuous variable age as cubic splines (25th, 75th percentile)?

[Ksharp · Accepted solution]

Check @Rick_SAS blogs for this topic:

https://blogs.sas.com/content/iml/2020/05/11/cubic-interpolation-sas.html
https://blogs.sas.com/content/iml/2017/04/19/restricted-cubic-splines-sas.html
https://blogs.sas.com/content/iml/2019/10/16/visualize-regression-splines.html
https://blogs.sas.com/content/iml/2024/06/03/vize-multivar-regression-splines.html
https://blogs.sas.com/content/iml/2019/02/18/regression-restricted-cubic-splines-sas.html

Here I use the code written by Rick to get cubic spline effect/term , if you want cubic interpolation check the first one URL I marked bold.

data cars;
set sashelp.cars;
keep mpg_city weight;
run;
proc glmselect data=cars outdesign(addinputvars fullmodel)=SplineBasis;
   effect spl = spline(weight / naturalcubic basis=tpf(noint) knotmethod=percentilelist(25 50 75) );
   model mpg_city = spl / selection=none noint;  
quit;