This is a classic application of quantile regresion. There are several examples of doing this in the doc for the QUANTREG procedure. For a parametric model, see http://support.sas.com/documentation/cdl/en/statug/63962/HTML/default/viewer.htm#statug_qreg_sect006.htm or http://support.sas.com/documentation/cdl/en/statug/63962/HTML/default/viewer.htm#statug_qreg_sect035.htm For a nonparametric spline model (which uses the new EFFECT statement), see http://support.sas.com/documentation/cdl/en/statug/63962/HTML/default/viewer.htm#statug_qreg_sect034.htm ... View more