How good is one teacher compared with another?

A growing number of school districts have adopted a system called value-added modeling to answer that question, provoking battles from Washington to Los Angeles  with some saying it is an effective method for increasing teacher accountability, and others arguing that it can give an inaccurate picture of teachers’ work.

The system calculates the value teachers add to their students’ achievement, based on changes in test scores from year to year and how the students perform compared with others in their grade.

People who analyze the data, making a few statistical assumptions, can produce a list ranking teachers from best to worst.

Use of value-added modeling is exploding nationwide. Hundreds of school systems, including those in Chicago, New York and Washington, are already using it to measure the performance of schools or teachers. Many more are expected to join them, partly because the Obama administration has prodded states and districts to develop more effective teacher-evaluation systems than traditional classroom observation by administrators.