A national study by the Bill & Melinda Gates Foundation found that effective teaching can be measured, adding weight to the arguments of those who support using students' test scores to evaluate a teacher's performance.

The results come as the clock is ticking on a teacher evaluation deal between New York City's Department of Education and its teachers' union, with negotiations stuck largely over this issue of "value-added" metrics.

"We are confident that we can identify groups of teachers who are comparatively more effective than their peers in helping students learn," the researchers said. "Great teaching does make a difference."

The report was released by the Measures of Effective Teaching (MET) project, which relied on 3,000 teacher volunteers from New York City and five other public school systems, including Denver, Colorado and Charlotte-Mecklenburg, North Carolina.

The $45 million, three-year project sought to answer a nagging question: do some teachers seem more effective just because they have higher performing students?

Researchers produced estimates of teacher effectiveness for each participant based on student test scores on state exams in 2009-10. They adjusted to account for differences in their students' prior test scores, racial and ethnic demographics and poverty. Each teacher was then randomly assigned a whole classroom of students in 2010-11 to see what would happen once students were evenly distributed - without the bias of principal selection.

Thomas Kane, the study's principal investigator and a professor of education and economics at the Harvard Graduate School of Education, said he normally avoids the word "cause" because of his academic training. However, he said, "We found that teachers that were more effective at promoting achievement on the state test also caused students to score higher on other more challenging assessments in math and in English."

The research project also looked at how much weight should be given to different measurements of teacher effectiveness. It found student test scores on state exams should count for 33-50 percent of a teacher's rating. And it said student test scores are a more effective indicator of teacher effectiveness than either teacher experience or education.

But it also recommended at least two annual classroom observations by different trained observers. It found video recordings of lessons were useful and could lower the cost of additional observations, if necessary. It also recommends using student surveys.

New York State's teacher evaluation law allows state test scores to count for up to 40 percent of a teacher's evaluation. Classroom observations make up the rest. Teachers must be notified in advance about one of those visits, and the observers must be trained and certified. However, both observations may be done by the same person (a principal, for example), although school districts can choose to expand upon that legal minimum.

The study's findings are unlikely to have any impact on negotiations over a new teacher evaluation system for New York City, which must be agreed upon by Jan. 17 in order for the city to receive $250 million in state aid. Reaction to the study fell along predictable lines. Those who support using student test scores found evidence to fuel their arguments, and so did skeptics.

New York City Schools Chancellor Dennis Walcott said:

"This report outlines exactly what the city has sought for our teachers and students: a fair evaluation system that looks at many factors, like classroom observations and student achievement. The study shows that evaluation systems can help teachers grow and learn – which in turn helps our students succeed."

But American Federation of Teachers president Randi Weingarten, whose members in New York and other cities participated in the project, issued her own victorious statement.

“The Gates Foundation’s study makes clear that evaluation of teachers must start with genuine feedback, which means the days of haphazard or check-list observation of teachers must end. Just dropping by a teacher’s classroom and writing up an evaluation must be replaced with a more serious process that actually helps improve teacher practice and student learning."

New York City's United Federation of Teachers wants principals to meet with teachers prior to their observations, and claims the city plans to use its evaluation system to punish teachers instead of helping them improve. The city has accused the union of adding extra, unrelated demands to the negotiations.

Eric Hanushek, senior fellow at the Hoover Institution at Stanford University, predicted the study would influence school systems across the country as they develop new teacher evaluation systems with financial encouragement by the Obama administration. "It gives a lot more weight to those who want to have combined measures of performance," he said. "Combined measures that include student test performance and other external evaluations."

Aaron Pallas, a professor of sociology and education at Teachers College-Columbia University, agreed. However, he said, there are trade-offs when designing the perfect formula for grading a teacher.

"Some observational schemes are shakier than others, with a single full-length observation by a principal the least reliable of the approaches considered," he said. "And nobody knows how any of this will work when teacher evaluation systems are operating under high-stakes conditions."

Another academic, education professor Bruce Baker at Rutgers University, noted that New York City's first attempt at using student test scores to measure teacher effectiveness had huge margins of error.

"My biggest issue is that this report and others continue to think about this all wrong," he said. "Yes, the information might be useful, but not if forced into a decision matrix or weighting system that requires the data to be used/interpreted with a level of precision or accuracy that simply isn't there - or worse - where we can't know if it is."

When asked if the new report would have any influence on districts such as New York, Vicki Phillips, the director of College Ready, Education for the Gates Foundation, said "We're not trying to settle political debates, that's not what this is about. We're trying to follow the evidence."