- A Princeton economist, Alan B. Krueger, took the offer [of
raw data], and after two years recently concluded that Professor
Peterson had it all wrong -- that not even the black students using
vouchers had made any test gains. ...
What makes this a cautionary tale for political leaders seeking to draft public policy from supposedly scientific research is the mundane nature of the apparent miscalculations. Professor Krueger concluded that the original study had failed to count 292 black students whose test scores should have been included. And once they are added -- making the sample larger and statistically more reliable -- vouchers appear to have made no difference for any group.
Via the indispensible dsquared, which goes on to discuss why "data dredging" is an unsound research method even if the data you're using is something like accurate (in a post to which the links are now bloggered).