What Does 20k Buy?

New study from Mathematica, summarized on Hechinger Report:

Paying good teachers $20K to move to bad elementary schools works and is cheaper than reducing class sizes

A November 2013 Mathematica study conducted for the Institute of Education Sciences within the U.S. Department of Education shows that paying good teachers $20,000 to transfer to a low performing elementary school raised the test scores of students by 4 to 10 percentile points. No positive effect was found at the middle school level.

Mathematica found that the same test score increases could be achieved by reducing class sizes and filling the teacher vacancies as usual. But it’s more cost effective to pay the bonuses. “The cost savings could be as large as $13,000 per grade at a given school,” according to the report. Furthermore, 60 percent of the 81 teachers in the study stayed at the low-performing schools even after the bonus payments ended.

I think they buried the lede a little.   Even $20k didn't move many teachers, at least in this particular context.  81 out of 1,500 agreed to change schools for the money. 

Another thought: this strategy is zero sum, right?  Charlotte is redistributing 81 proven teachers, and harming the children from their "former" schools? 

This policy is different from, say, wealth transfer.  In schooling, since the correlation of money to learning is weak, you can't say spending less on some schools and more on others causes clear harm.  But with top teacher redistribution, you can.  There's data showing that good teachers, when they change schools, stay good. 

The policy seems different from high-performing charters, too.  (Remember usual caveat: typical charter in USA not so hot.)  When high-performing charters hire TFA alums who'd had success in district schools, typical alternative that those teachers describe is that they'd have left teaching entirely.  So they're not redistributing.  (Also, typically these teachers take a pay cut to go to a charter, not a pay raise.  Asked why they discount themselves, it's typically to get a positive school culture, and a community of other teachers who are "rowing in the same direction," while still serving similar -- though not identical -- students.)