Paul Krugman apparently has gotten caught massaging the statistics on Wisconsin education versus Texas when he was trying to show that collective bargaining provides better schools. Krugman argues that, overall, Wisconsin students perform better than Texas students on national tests....
However, Krugman had "forgotten" to factor in the racial composition of the states. Some call it cherry-picking, Falkenblog calls it Simpson's Paradox.
Iowahawk states it plain: "white students in Texas perform better than white students in Wisconsin, black students in Texas perform better than black students in Wisconsin, Hispanic students in Texas perform better than Hispanic students in Wisconsin."
This conclusion was corroborated by some dude at Harvard for those more comfortable with an east coast authority figure.