A growing number of school districts are adopting "value-added" measures of teaching quality to award bonuses or even tenure. And two competitive federal grants are spurring them on. Districts using value-added data are encouraged by the results. But researchers who support value-added measures advise caution. The ratings, which use a statistical method to judge teachers' contributions to their students' test scores, can't compensate for an otherwise weak teacher evaluation system, they say. The federal government only awards its Race to the Top grant to states that permit using student achievement data to evaluate teachers, even while experts debate whether student test scores can be used legitimately for that purpose. Meanwhile, journalists have been accused of adding fuel to the controversy by publishing value-added rankings of individual teachers.