Here is a common pattern: If you do 20 studies comparing the effect that A has on B, you may find that in 18 of those studies there is no correlation between A and B. In one of the other two, you may find that more A is followed by more B. And in the other, more A is followed by less B. Overall, still no correlation.
Depending on what the education establishment wants, they can seize upon the one study out of 20 that showed more A leading to more B and burst into the media with it. If the conclusion of that one study fits in with the media vision of the world, then it may be trumpeted across the land as "proof."
The Head Start program is a classic example. Anyone who expresses any skepticism about claims that Head Start is a great success will be denounced as someone who doesn't "care" about the low-income and minority children that this program supposedly helps. One of the great propaganda tricks is to change questions of fact into questions of motives.
The Thernstroms show what feeble facts there are behind this program that has cost billions of dollars. Look for them to be denounced for being heartless, if not racist. But don't expect advocates of Head Start to engage in a serious discussion of facts.
It is much the same story when it comes to claims that "studies prove" that small classes lead to better education. The Thernstroms show cases where class sizes as small as 12 led to no better results when the students were tested.
Ordering students bussed from their own neighborhoods for the sake of racial balance has similarly failed to produce the much-trumpeted educational benefits.
The time is long overdue to start looking at facts instead of listening to rhetoric. Reading "No Excuses" is a start.