It all makes sense now.
At first I was a bit surprised that Bill Gates and Michelle Rhee were opposed to publicizing the value-added data from New York City and other cities.
Could they be experiencing twinges of a bad conscience?
That’s not it. Nor do these educational Deformers think that value-added mysticism is nonsense. They think it’s wonderful and that teachers’ ability to retain their jobs and earn bonuses or warnings should largely depend on it.
The problem, for them, is that they don’t want the public to see for themselves that it’s a complete and utter crock. Nor to see the little man behind the curtain.
I present evidence of the fallacy of depending on “value-added” measurements in yet another graph — this time using what NYCPS says is the actual value-added scores of all of the many thousands of elementary school teachers for whom they have such value-added scores in the school years that ended in 2006 and in 2007.
I was afraid that by using the percentile ranks as I did in my previous post, I might have exaggerated or distorted how bad “value added” really was.
No worries, mate – it’s even more embarrassing for the educational deformers this way.
In any introductory statistics course, you learn that a graph like the one below is a textbook case of “no correlation”. I had Excel draw a line of best fit anyway, and calculate an r-squared correlation coefficient. Its value? 0.057 — once again, just about as close to zero correlation as you are ever going to find in the real world.
In plain English, what that means is that there is essentially no such thing as a teacher who is consistently wonderful (or awful) on this extremely complicated measurement scheme. How teacher X does one year in “value-added” in no way allows anybody to predict how teacher X will do the next year. They could do much worse, they could do much better, they could do about the same.
Even I find this to be an amazing revelation. What about you?
And to think that I’m not making any of this up. (unlike Michelle Rhee, who loves to invent statistics and “facts”.)
I neglected to give the links to where you can find the raw data. (Warning: some of these spreadsheets are enormous); Here they are: