Must value-added models grade teachers on a curve?
Add one more point of critique to New York City’s Teacher Data Reports: experts and educators are worried about the bell curve along which the teacher ratings fell out.
Like the distribution of teachers by rating across types of schools, the distribution of scores among teachers was essentially built into the “value-added” model that the city used to generate the ratings.
The long-term goal of many education reformers is to create a teaching force in which nearly all teachers are high-performing. However, in New York City’s rankings—which rated thousands of teachers who taught in the system from 2007 to 2010—teachers were graded on a curve. That is, under the city’s formula, some teachers would always be rated as “below average,” even if student performance increased significantly in all classrooms across the city.
The ratings were based on a complex formula that predicts how students will do—after taking into account