Fuzzy Math

The guesstimate that struck down California’s teacher tenure laws.

…testified that 1 to 3% of teachers in California are grossly ineffective. Given that the evidence showed roughly 275,000 active teachers in this state, the extrapolated number of grossly ineffective teachers ranges from 2,750 to 8,250.

But where did this number come from?
Nowhere, it turns out. It’s made up. Or a “guesstimate,” as David Berliner, the expert witness Treu quoted, explained to me when I called him on Wednesday. It’s not based on any specific data, or any rigorous research about California schools in particular. “I pulled that out of the air,” says Berliner…

The expert cited in the ruling doesn’t even necessarily believe that low test scores qualify somebody as a bad teacher. Read this article in its entirety at Slate.com