Value-added estimates of teachers' contributions to student achievement have been criticized for bias relating to the sorting of students to classrooms. More recently, research has raised the possibility that sorting leads to differences in practice evaluation ratings between teachers of more or less disadvantaged and/or higher- and lower-achieving students. Adjusting observation ratings for the relevant characteristics of teachers' classrooms has been proposed as a remedy, analogous to how value-added teacher effectiveness estimates are developed. However, the appropriateness of adjustment depends on the cause of observed differences in average ratings and the use of the ratings. Potential causes include rater bias rubric deficiency, differences in teacher skills and beliefs, and true differences in difficulty of teaching groups of students. The appropriateness of adjustment given these causes and typical uses of the ratings is discussed as well as research needed to identify the influence of the causes.