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A growing number of school districts have adopted a system called value-added modeling to laser cutting equipmentthe answer that question, provoking battles from Washington to Los Angeles — with some saying it is an effective method for increasing teacher laser cutting system 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 laser machinefrom year to year and how the students perform compared with others in their grade.People who analyze the data, making a few statistical laser cutting ma 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 laser cutter to develop more effective teacher-evaluation systems than traditional classroom observation by administrators.who gets fired.Michelle A. Rhee, the schools laser cutting chancellor in Washington, fired about 25 teachers this summer after they rated poorly in evaluations based in part on a value-added analysis of scores.And 6,000 elementary school teachers in Los Angeles have found themselves under scrutiny this summer after The Los Angeles Times published a about their performance, including a searchable laser cutting machinedatabase on its Web site that rates them from least effective to most effective. The teachers’ union has protested, urging a boycott of the paper.Education Secretary weighed in to support the newspaper’s work, calling it an laser cutter exercise in healthy transparency. In a speech last week, though, he qualified that support, noting that he had never released to news media similar information on teachers when he was the Chicago schools superintendent.“There are real issues and competing priorities and values that we must work through laser engraver together — balancing transparency, privacy, fairness and respect for teachers,” Mr. Duncan said. On The Los Angeles Times’s publication of the teacher data, he added, “I don’t advocate that approach for laser engraving machine other districts.”A released this month by several education researchers warned that the value-added methodology can be unreliable.“If these teachers were measured in a different year, or a different model were used, the rankings might bounce around quite a bit,” said Edward Haertel, a Stanford professor who was a co-author of the laser engraving machine report. “People are going to treat these scores as if they were reflections on the effectiveness of the teachers without any appreciation of how participated laser engraving unstable they are.”Other experts disagree.William L. Sanders, a senior research manager for a North Carolina company, SAS, that does value-added estimates for districts in North Carolina, Tennessee and other states, said that “if you use rigorous, robust methods and surround them with safeguards, you can reliably distinguish highly effective teachers from average teachers and from ineffective teachers.”Dr. Sanders laser engraving generals helped develop value-added methods to evaluate teachers in Tennessee in the 1990s. Their use mini laser engraver spread after the 2002law required states to test in third to eighth grades every year, giving school districts mountains of test data that are the raw material for value-added analysis.In value-added modeling, researchers use students’ scores on state tests administered at the end of third grade, for instance, to predict how they are likely to score on state tests at the end of fourth grade.A student vinyl cutter whose third-grade scores were higher than 60 percent of peers statewide is predicted to score higher than 60 percent of fourth graders a year later.If, when actually taking the state tests at the end of fourth grade, the student scores higher than 70 percent of fourth graders, the leap in achievement represents the value the desktop laser engraver fourth-grade teacher added. Even critics acknowledge that the method can be more accurate for rating schools than the system now required by federal law, which compares test scores of succeeding classes, for instance this year’s fifth graders with last year’s fifth graders. But when the method is used to evaluate individual teachers, many factors can lead to inaccuracies. Different people crunching the numbers can get different results, said Douglas N. Harris, an education professor at the University of Wisconsin, Madison. For example, two vinyl cutter analysts might rank teachers in a district differently if one analyst took into account certain student characteristics, like which students were eligible for free lunch, and the other did not.Millions of students change classes or s