Case-Mix Adjustment: Making Bad Apples Look Good

To the Editor. —Dr Salem-Schatz and colleagues have done admirable work investigating the influence of case-mix adjustment on physician profiles. 1 Readers should be aware, however, that case-mix adjustment may make bad apples look good. It is important to step back and consider the rationale for adjusting physician profiles in general. Adjustment is a mechanism to control for factors that confound the relationship between physician and practice style. Ideally, the intent is to remove the effect of exogenous factors (ie, those in which the physician has no influence). Age and sex adjustment serves as the classic example. On the other hand, it is important not to adjust away endogenous factors (ie, those that the physician can influence). For example, it would be silly to adjust physician profiles for physician income. In the private sector, at least, one would be adjusting for a factor that is strongly related to what one