About 6,000 Los Angeles elementary school teachers and 470 elementary schools are included in The Times' database of "value-added" ratings.
Third-, fourth- and fifth-grade teachers who taught at least 60 students from the 2002-03 through 2008-09 academic years were evaluated in the Times analysis. Most of Los Angeles Unified School District's elementary schools are included. Test scores for charter schools that do not report directly to the district were not available.
A teacher's value-added rating is based on his or her students' progress on the California Standards Tests for English and math. The difference between a student's expected growth and actual performance is the "value" a teacher added or subtracted during the year. A school's value-added rating is based on the performance of all students tested there during that period.
Although value-added measures do not capture everything that goes into making a good teacher or school, The Times decided to make the ratings available because they bear on the performance of public employees who provide an important service, and in the belief that parents and the public have a right to the information.