While debate rages in the education world about how to measure effective teaching - or whether it is even possible to do so - research funded by a prominent advocate of data-driven analysis has found that growth in annual student test scores is a reliable sign of a good teacher.

The Bill and Melinda Gates Foundation reported that and other preliminary findings Friday from a $45 million study of teacher effectiveness in several cities.

"In every grade and subject we studied, a teacher's past success in raising student achievement on state tests was one of the strongest predictors of his or her ability to do it again," said Vicki Phillips, who oversees elementary and secondary education grants for the foundation.

The findings arrive as state and local officials across the country are retooling teacher evaluations in an effort to make the annual ratings of teachers more objective than the customary short classroom observations made by a principal.

In many cases, officials are moving toward "value-added" analysis - that is, the growth in achievement that occurs in a class from one school year to another, measured in large part through gains on state tests. There are many issues with how such analysis is conducted. Often teachers are not assigned to subjects that a state assesses. Students might rotate among teachers on a daily or weekly basis for certain lessons.

At certain ages and in certain circumstances - which can vary wildly depending on family background - students might be more or less ready to have an academic growth spurt.

Teachers unions and other skeptics have cited these and other arguments as reasons to move cautiously on value-added analysis. But in D.C. public schools and many others, growth in test scores is now an ingrained part of teacher evaluation and pay. President Obama has encouraged this movement through the $4 billion Race to the Top education grants.

The preliminary Gates findings are based on test data and student surveys from public school systems in New York, Dallas, Denver, Charlotte-Mecklenburg, N.C., and Hillsborough County, Fla. Also participating in the study are Memphis and Pittsburgh schools. Researchers were drawn from the Educational Testing Service and several major schools, including Harvard and Stanford universities and the University of Virginia.

The central finding indicates that teachers with "value-added" ratings are able to replicate that feat in multiple classrooms and in multiple years.

The foundation in the past year has collaborated with local teachers unions on reshaping teacher pay and evaluation in several major school systems.

A final report is due the winter of 2011-12.