A new study of teacher evaluation finds that the more that value-added test scores count, the lower teachers are rated.

“Among the most important findings from this new study: When value-added scores are incorporated into evaluations, the ratings tend to go down. And the more weight a system puts on value-added scoring, the lower the scores are likely to be, the study showed.

“That’s because value-added scores tend to be relative measures, explained Kraft. Value added is generally “designed to just compare you to your peers,” he said. “Everybody can’t be good with value added.”

“On the other hand, with classroom observation scores, everyone can be excellent, he said.

“And that leads to a huge caveat in all of this: As it stands, despite the variation in systems, almost all teachers across the country continue to get positive ratings.

“That’s largely because observation scores make up the meat of most teacher evaluation systems, said Kraft. And as we know from previous research, principals tend to rate their teachers highly.”