For years, schools and students have been judged on raw standardized test scores. Experts say this approach is flawed because they tend to reflect socioeconomic levels more than learning.
The “value-added” approach attempts to level the playing field by focusing on growth rather than achievement. Using a statistical analysis of test scores, it tracks an individual student’s improvement year to year, and uses that progress to estimate the effectiveness of teachers, principals and schools.
Academics have also used the approach to test many assumptions about what matters in schools. Scholars are still puzzling over what makes for a great teacher or school, but their results challenge orthodox assumptions like these:
All teachers are equal. For decades, schools have treated teachers like interchangeable parts. Value-added results suggest there are sharp differences in teachers’ effectiveness.