As a result, the state has moved to a system where "value added" is measured for each teacher. This approach looks at CSAP improvement of children in a class during the course of a class. Are students improving more than average, less than average or typically?
Clearly, this is seems more fair. At the very least, it disentangles a lot of what happened before a child arrived in a teacher's class. But, does it really measure teaching quality?
A new twin study of U.K. twins finds that academic improvement measures like the system used in Colorado also have significant genetic components.
"These findings do not mean that educational quality is unimportant, in fact environmental factors were just as important as genetic factors. However, these results do suggest that children bring characteristics to the classroom that influence how well they will take advantage of the quality."
Surprisingly, according to the open access study, value added may actually be worse at assessing teacher performance than raw achievement scores:
Raw achievement shows moderate heritability (about 50%) and modest shared environmental influences (25%). Unexpectedly, we show that for indices of the added value of school, genetic influences remain moderate (around 50%), and the shared (school) environment is less important (about 12%). . . . At first glance, this high degree of genetic overlap between different cognitive and academic measures suggests that correcting achievement measures for general cognitive ability would remove the genetic influence on achievement. However, this genetic overlap is not 100%, so there could be residual genetic influences on achievement that are independent of those on general cognitive ability, or even previous measures of achievement. . . . The results were striking, indicating that even when previous achievement and a child's general cognitive ability are both removed, the residual achievement measure is still significantly influenced by genetic factors (heritabilities of 48% and 37% respectively for teacher-ratings and test data). The main point, to which we shall return, is that corrected-achievement scores are influenced by genetic factors that are independent of those influencing g or previous achievement.
Of course, great individual differences in educational improvement ability don't necessarily detract from a value added measure of teacher evaluation based on test scores if the mix in any given class of thirty kids in a classroom, or hundreds of kids in a school, tends to average out.
We know that raw achievement scores have a strong socio-economic component that varies greatly from school to school. We don't know, and the study doesn't tell us, if this achievement controlled improvement factor varies in a systemic way that is likely not to average out from school to school. For example, the study doesn't tell us, and U.K. data would probably not be very helpful in any case in measuring, if there were strong ethnic components to that variation.
If value added components are strongly heritable, but the genetic component of a value added effect controlled for raw achievement is randomly distributed from one classroom to the next, a value added measure is still valid.
But, if there is a clear pattern in which some category of students routinely improves a lot even after controlling for past achievement, while another category of students routinely improves little, then the new measure may simply create a more subtle version of the problem that causes Colorado to go from absolute performance to value added based evaluations of teachers and schools in the first place.
Eyeballing the latest round of Colorado value added evaluations data, the only distinct trend that I noted was that schools with a lot of English language learners seemed to have an edge in value added measures, presumably because poor English language skills of smart students suppressed their prior year achievement, but this effect presumably rapidly eased as the students gained English language mastery.
Colorado's system also addresses concerns that learning disabilities might have an effect independent of raw achievement levels, by limiting the way that the system can be used in classes with many special education students.