Dear Dr. Rick Wicklin,

For the model of restricted cubic spline, in Frank E. Harrell's (2015) book, he mentioned that the reduced form of restricted cubic spline formula is

f(x) = intercept + linear *x + B2 (x-t1)^3 + B3 (x-t2)^3...

However, in your predicted value: Pred = 34.96 – 5*Spl_1 + 2.2*Spl_2 – 3.9*Spl_3

I did not see any cubic polynomial in this predicted value.

Is SAS using a different method to calculate the restricted cubic spline?

Furthermore, for Pred = 34.96 – 5*Spl_1 + 2.2*Spl_2 – 3.9*Spl_3,

Should this be Pred = 34.96 – 5*x + 2.2*x – 3.9*x ?