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.

This weekend, The Times will examine how a bruising value-added debate played out in San Diego, offering a preview of a controversy likely to sweep the nation. Read the details here: Educator sees the value in 'value-added' approach to evaluating teachers

A new way of crunching test scores is turning conventional ideas in education on their head. The approach, called value added, has gained momentum in recent months as it has been embraced by the Obama administration and policymakers around the country, though it has generated strong opposition from teachers unions.

The "value-added" approach attempts to level the playing field by focusing on growth rather than achievement. Using a complex statistical analysis of test scores, it tracks an individual student's improvement year to year, and uses that progress to estimate the effectiveness of individual teachers, principals and schools.

Academics also have used the approach to test many assumptions about what matters in schools. While the scholars are still puzzling over what makes a great teacher or school, their results challenge many of the things once assumed important:

All teachers are equal. For decades, schools have treated teachers like interchangeable parts. Value-added results suggest that there are dramatic differences in the effectiveness of teachers.

More learning happens in wealthy schools. The highest growth among students is often in poor schools with low achievement scores, according to results at districts and states that have adopted the value-added approach. Students at affluent schools at times have high proficiency scores but make little new progress year to year.

Teachers can't overcome a student's background. Recent research shows that with several effective teachers in a row, students can overcome the disadvantages they bring to the classroom. Some studies suggest that minority and poor students make as much progress as other students when placed with the same effective teachers.

Class size is key. Research suggests that modest changes in class size, such as decreasing it by four or five students, has been shown to have little to no effect on student learning.

Bad teachers tend to teach in poor schools. Several studies suggest that there is more variation among teachers within a school that across schools. Effective instructors often are distributed across rich and poor schools, and they tend to stay in challenging schools longer than ineffective ones.

Teacher experience matters. Although teachers are generally paid more for years of experience, research suggests that instructors show dramatic improvement in their first few years and then level off. Teachers with 20 years of experience are often no more effective than peers with five years.

Teacher education matters. Schools routinely pay teachers higher salaries for obtaining master's degrees. But several studies have found that educators with advanced degrees do no better than those without (with the possible exception of high school math teachers).

Teacher credentials matter. Most public schools pay teachers more for certifications and advanced credentials. But several studies have shown that non-traditionally prepared instructors — such as those in Teach for America — have similar or slightly better outcomes to certified ones.

—Jason Song and Jason Felch