Schools Chancellor Joel Klein has for years been a proponent of paying some teachers more based on their performance, and he has made some headway in introducing merit pay in the city schools. But the policy has plenty of critics, from teachers who say merit pay divides them to statisticians who point simply to flaws in the measures on which pay calculations are based.

In the video above, University of Virginia psychology professor Dan Willingham gives six reasons in three minutes why paying teachers based on their students’ test scores isn’t statistically sound. But Willingham doesn’t totally rule out the prospect of paying better teachers more: “Merit pay can’t work until there’s a way to measure teacher performance that’s fair,” he concludes.