Models for livestock breeding
One of the early motivating applications for linear mixed models was agricultural field studies looking at animal breeding or plant breeding. These are statistical models that combine differences between groups of observations with correlations between similar observations in order to get better comparisons.
John Oliver’s “Last Week Tonight” argues that these models shouldn’t be used to evaluate teachers , because they have been useful in animal breeding (with suitable video footage of a bull mounting a cow). It’s really annoying when someone bases a reasonable conclusion on totally bogus arguments.
As the American Statistical Association has said on value-added models for teaching (PDF), the basic idea makes some sense, but there are a lot of details you have to get right for the results to be useful. That doesn’t mean rejecting the whole idea of considering the different ways in which classes can be different, or giving up on averages over relevant groups. On the other hand, the mere fact that someone calls something a “value-added model” doesn’t mean it tells you some deep truth.
It would be a real sign of progress if we could discuss whether a model adequately captures educational difficulties due to deprivation and family advantage without automatically rejecting it because it also applies to cows, or without automatically accepting it because it has the words “value-added.”
But it probably wouldn’t be as funny.
Thomas Lumley (@tslumley) is Professor of Biostatistics at the University of Auckland. His research interests include semiparametric models, survey sampling, statistical computing, foundations of statistics, and whatever methodological problems his medical collaborators come up with. He also blogs at Biased and Inefficient See all posts by Thomas Lumley »
But it’s not hard to see that the questions – “how much milk is my cow producing?” is a very different question to “how much learning has my child achieved?”
The first question has a clear and easily measured response. The second question would lead to a long debate about the meaning and purpose of education, followed by an even longer debate about how such learning could be measured.
8 years ago
On an unrelated note, are you planning on crunching the numbers for super rugby again? Starts next week…
8 years ago