How does SAS compute the vertical axis component of the selected knots. I know that SAS will fit a cubic b-spline basis with three equally spaced knots positioned between the maximum and minimum values of the x-axis. My sense is that the y-axis value for a knot is determined by finding the y-axis value that equates to the desired quantile position in the data "near" the knot. Thereafter, the splines can be fit between the knots. I truly appreciate any information that you can share with me.