Both research and practice of value-added models (VAM) have been growing in recent years due to the widespread effort to quantify teacher effectiveness. The existing VAM literature has not yet tested the sensitivity of value-added estimates to the rules that define which students contribute to each teacher’s value-added estimate. Student-teacher linkages are often a complex network due to various transfers, students taking multiple courses in the same subject, and students receiving special education or other “pull-out” services. Complex linkages are often considered as among the main threats to the validity of VAM. In this paper we conducted a case study to examine the sensitivity of VAM to the alternative link definitions. We examined three popular VAM approaches and applied alternative rules for linking students to teachers with each method. We found no overall sensitivity of estimated teacher effects to the linking rules. Even though more teachers had value-added estimates under more inclusive rules, for a teacher with estimates under all three rules, the correlation among pairs of estimates created using different linking rules was always above 0.95 and generally above 0.98 for each VAM approach. The value-added estimates of a small number of teachers were affected by highly different link definitions and these tended to be teachers with small numbers of students. Restricting the minimal sample size for calculating individual teachers’ value-added largely reduced the sensitivity in link definition.
©2012 Walter de Gruyter GmbH & Co. KG, Berlin/Boston