Random Assigment and Bias in VAM Estimates – Article Published in AERJ

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“Nonsensical,” “impractical,” “unprofessional,” “unethical,” and even “detrimental” – these are just a few of the adjectives used by elementary school principals in Arizona to describe the use of randomized practices to assign students to teachers and classrooms. When asked whether principals might consider random assignment practices, one principal noted, “I prefer careful, thoughtful, and intentional placement [of students] to random. I’ve never considered using random placement. These are children, human beings.” Yet the value-added models (VAMs) being used in many states to measure the “valued-added” by individual teachers to their students’ learning assume that any school is as likely as any other school, and any teacher is as likely as any other teacher, to be assigned any student who is as likely as any other student to have similar backgrounds, abilities, aptitudes, dispositions, motivations, and the like.

One of my doctoral students – Noelle Paufler – and I recently reported in the highly esteemed American Educational Research Journal the results of a survey administered to all public and charter elementary principals in Arizona (see the online publication of “The Random Assignment of Students into Elementary Classrooms: Implications for Value-Added Analyses and Interpretations”). We examined the various methods used to assign students to classrooms in their schools, the student background characteristics considered in nonrandom placements, and the roles teachers and parents play in the placement process. In terms of bias, the fundamental question here was whether the use of nonrandom student assignment practices might lead to biased VAM estimates, if the nonrandom student sorting practices went beyond that which is typically controlled for in most VAM models (e.g., academic achievement and prior demonstrated abilities, special education status, ELL status, gender, giftedness, etc.).

We found that overwhelmingly, principals use various placement procedures through which administrators and teachers consider a variety of student background characteristics and student interactions to make placement decisions. In other words, student placements are by far nonrandom (contrary to the methodological assumptions to which VAM consumers often agree).

Principals frequently cited interactions between students, students’ peers, and previous teachers as justification for future placements. Principals stated that students were often matched with teachers based on their individual learning styles and respective teaching strengths. Parents also yielded considerable control over the placement process with a majority of principals stating that parents made placement requests, the majority of which are often honored.

In addition, in general, principal respondents were greatly opposed to using random student assignment methods in lieu of placement practices based on human judgment—practices they collectively agreed were in the best interest of students. Random assignment, even if necessary to produce unbiased VAM-based estimates, was deemed highly “nonsensical,” “impractical,” “unprofessional,” “unethical,” and even “detrimental” to student learning and teacher success.

The nonrandom assignment of students to classrooms has significant implications for the use of value-added models to estimate teacher effects on student learning using large-scale standardized test scores. Given the widespread use of nonrandom methods as indicated in this study, however, value-added researchers, policymakers, and educators should carefully consider the implications of their placement decisions as well as the validity of the inferences made using value-added estimates of teacher effectiveness.

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