Breaking news: If you base teacher layoffs on seniority rather effectiveness, you wind up firing some really good teachers and keeping some teachers who are pretty meh.

OK, that's not breaking news. But a new study by Dan Goldhaber and Roddy Theobald from the University of Washington does the important and depressing work of quantifying just how many effective teachers (as measured by value-added scores) are getting the boot—and how many senior teachers are handing around until their pensions kick in—thanks to powerful teachers unions and the "last in, first out" policies they favor.

The overlap between the subgroup of teach­ers who received a layoff notice and the sub­group of teachers who received one in our simu­lation is relatively small—only 23 teachers (or 16 percent of the teachers for whom we could estimate value-added who received a layoff notice)…. As expected, there are large differences in classroom effec­tiveness between teachers who actually received layoff notices and those who would have received them in our effectiveness-based simulation. The two groups differ by about 20 percent of a standard deviation in students' math and reading achieve­ment. The magnitude of the difference is strik­ing, roughly equivalent to having a teacher who is at the 16th percentile of effectiveness rather than at the 50th percentile. This difference corresponds to roughly 2.5 to 3.5 months of student learning.

There's also the question of sheer quantity of teachers. If you lay off newer teachers with smaller salaries, you have to lay off more of them to reach your budget goals:

The authors find that if the RIF-notified teachers made the average salary in their district, it would only be necessary to lay off 1,349 teachers [instead of 1,717 teachers] in order to attain the same budgetary savings, or roughly 20 percent less than the actual number of teachers who received layoff notices.

Via Education Next.