Proc Transreg looks like a powerful tool but it seems impossible to figure out the estimated model. From the SAS example I added "DETAIL" but only get knots and coefficients and can't relate them back to the transformation which I assume is cubic by default. The example below has 3 knots and therefore 4 cubic equations i.e. 12 coefficients plus an intercept but the output shows 7 coefficients. What am I doing wrong? is there a paper on this someplace? thanks for any help you can give. Joe.
title2 'A Cubic Spline Fit with Knots at X=5, 10, 15';
proc transreg detail data=a;
model identity(y) = spline(x / knots=5 10 15);
output out=trans coefficients;
proc print;
run;