Los Angeles will forge ahead with a plan to base teacher pay in part on "value-added" testing, a measure of progress students make in teachers' classes. The Los Angeles Times reports. It sounds simple, but it isn't, beginning with obvious problems in student raw material (children who've transferred; poor English speakers; concentrations of at-risk kids), students taught by multiple teachers and many other complications.
This alone gives you some idea:
"It is too unreliable when you're talking about messing with someone's career," said Gayle Fallon, president of the Houston Federation of Teachers.
She said many teachers don't understand the calculations. The general formula for the "linear mixed model" used in her district is a string of symbols and letters more than 80 characters long:
y = Xβ + Zv + ε where β is a p-by-1 vector of fixed effects; X is an n-by-p matrix; v is a q-by-1 vector of random effects; Z is an n-by-q matrix; E(v) = 0, Var(v) = G; E(ε) = 0, Var(ε) = R; Cov(v,ε) = 0. V = Var(y) = Var(y - Xβ) = Var(Zv + ε) = ZGZT + R.
"It's doctorate-level math," Fallon said.
Advocates promise that the complex formulas neutralize traditional testing score differences related to race and poverty. But do they?
A 2010 study of 3,500 students and 250 teachers in six Bay Area high schools by researchers at Stanford University and UC Berkeley found that, under their model, teachers with more African American and Latino students tended to receive lower value-added scores than those with more Asian students.
Test scores obviously matter. Individual progress obviously matters. Can it all be reduced to a grade for every teacher, from K through 12? Where are the break points on that grading scale? Lies, damn lies, statistics.