PRED v DV

From: LSheiner <lewis@c255.ucsf.edu> Subject: PRED v DV Date: Tue, 02 Feb 1999 12:36:54 -0800 Regarding the PRED vs DV debate: I am a little puzzled by Peter Bonate's emphasis on regression relationships. No one is going to regress PRED on DV or vice-versa: DV has presumably already been used as optimally as possible to generate PRED. The issue of errors in variables, which is admittedly important in estimating structural models, does not apply here since there is no structural model to be created. Rather, the sole issue is which plot serves the model diagnostic purpose better? As I remarked to Mats separately, there is no question that PRED is appropriate on the abscissa when, for example, residuals are to be plotted. In the case of PRED & DV, the choice is arbitrary, although as Mats point out, if one wants to put a smooth through the points, then since E(Y|PRED) should equal PRED, the mismatch of the smooth from the line of identity is diagnostic only when PRED is on the abscissa. Perhaps we have a tradition of DV on the abscissa because we are trying to see if there is something amiss with the model (PRED) conditional on the data (DV), and in regression, we tend to put the variable on which we are conditioning on the abscissa. LBS -- Lewis B Sheiner, MD Professor: Lab. Med., Biopharm. Sci., Med. Box 0626 UCSF, SF, CA 94143-0626 voice: 415 476 1965 fax: 415 476 2796 email: lewis@c255.ucsf.edu