Accounting for Co-Teaching: A Guide for Policymakers and Developers of Value-Added Models

We outline the options available to policymakers for addressing co-teaching in a value-added model. Building on earlier work, we propose an improvement to a method of accounting for co-teaching that treats co-teachers as teams, with each teacher receiving equal credit for co-taught students. Hock and Isenberg (2012) described a method known as the Full Roster Method (FRM) that is feasible and practical, but it effectively counts co-taught students more than once-these students receive a full weight with each of their teachers, so such students receive extra weight when calculating the relationship between student characteristics and achievement. The improvement, known as the Full Roster-Plus Method, allows co-taught students to receive full weight with their teachers, but all students contribute equally to the calculation of the relationship between student characteristics and achievement. To investigate how the application of this method empirically changes value-added estimates, we use data from District of Columbia Public Schools. We find that there are very small empirical differences between the two methods.