Often we hear that "all the experts agree" that A is better than B or that "studies prove" A to be better than B. But one of the reasons for this can be that only people who favor A over B are likely to get the money to conduct studies or be given access to the data needed for a study.

A few years ago, a book by William Bowen and Derek Bok paraded various statistics that they interpreted as proving the success of policies of preferential admission of blacks to colleges and universities.

A chorus of praise for this study was heard throughout the media and echoed in academia and among liberal politicians. The study was later cited in a landmark Supreme Court decision on affirmative action.

Not everyone thought this was a great study, however -- or even an adequate study. But no one was allowed access to the raw data on which the Bowen and Bok study was based. So no one else could run the numbers for themselves and reach their own conclusions.

Those who sought such data included Harvard professor Stephen Thernstrom, whose long and distinguished record of scholarship included being one of the creators of the Harvard Encyclopedia of American Ethnic Groups. He was refused access to the data.

When only people with one set of views are allowed to do certain studies, do not be surprised if "studies prove" that set of views is right.

I was not surprised that Professor Thernstrom was refused access to the data. I had had similar experiences.

Back in the 1970s, I tried to get statistical data from Harvard to test various claims about affirmative action. Derek Bok was then president of Harvard and he was the soul of graciousness, even praising a book on economics that I had written. But, in the end, I did not get to see one statistic.

During the same era I was also researching academically successful black schools. I flew across the country to try to get data on one school, talked with board of education officials, jumped through bureaucratic hoops -- and, after all this was done and the dust settled, I still did not get to see one statistic.

Why not? Think about it. Education officials have developed explanations for why they cannot educate black children. For me to write something publicizing outstanding academic results in this particular black school would be to open a political can of worms, leading people to ask why the other schools can't do the same.

Education bureaucrats decided to keep that can sealed.

Critics of affirmative action have long said that mismatching black students with colleges that they do not qualify for creates wholly needless academic failures among these students, who drop out or flunk out of colleges that they should never have been in, when most of them are fully qualified to succeed in other colleges.

Has the ending of preferential admissions in the University of California system and the University of Texas system led to a rise in the graduation rates of black students, as critics predicted? Who knows? These universities will not release those statistics.

This is not peculiar to the United States. In Britain, the claim has been repeated endlessly that putting criminals in prison "doesn't work" and that various rehabilitation programs "in the community" are more successful in reducing criminals' repetition of their crimes.

When statistical data from the Home Office showed the direct opposite of what was being proclaimed by the Home Secretary, other high officials, the media, and academics, the solution was simple: Such data were no longer released.

Sometimes it is not the data but the money that is used to limit who can do studies on controversial issues. Advocates of "global warming" have access to all sorts of government research money but skeptics and critics can depend on no such largess and may even be risking their careers by angering bureaucrats who have staked a lot on this crusade and who control the purse strings.

Even when the taxpayers' money is used to collect data or finance research, those who dispense that money and control that data often treat these things as if they were their own private property, to be used to promote research congenial to their own ideologies or interests.