You know that controversy over Michelle Rhee's firing of all those DCPS teachers recently using the new
IMPACT evaluation system? Oh, it was statistically unsound. Like, insanely unsound:
...Subtracting one score from another only makes sense if the two scores are on the same scale. We wouldn’t, for example, subtract 448 apples from 535 oranges and expect an interpretable result. But that’s exactly what the DC value-added approach is doing: Subtracting values from scales that aren’t comparable...
...Did DCPS completely botch the calculation of value-added scores for teachers, and then use these erroneous scores to justify firing 26 teachers and lay the groundwork for firing hundreds more next year?
According to the only published account of how these scores were calculated, the answer, shockingly, is yes.
The Answer Sheet