Last month in Perspective ("Oklahoma Preschool Study Provides No Evidence of Lasting Benefits") I pointed out that Oklahoma's NAEP scores suggest no return on the state's massive and celebrated investment in preschool over the past 18 years.

Oklahoma's 4th grade reading NAEP scores have dropped and stagnated compared to the national average, and changes in poverty levels and per-capita income can't explain why we don't see improvement from the state's model investment in preschool.

Regardless of the evidence that there is little or no long-term effect from preschool, the critics will always point to the remaining shadowy corners of uncertainty. With so many possible confounding variables, it is impossible to control for them all. There might be some hidden, overlooked factor that canceled out the real, substantial long-term effects.

That's correct. Highly unlikely in the case of the spectacular absence of a return on Oklahoma's preschool investment and no obvious alternative explanation. But possible nonetheless.

And that is why non-experimental analysis can only provide suggestive evidence, with a heavy dose of uncertainty. Among the available research methods, the only way to be fairly certain an educational treatment has had an effect on students is to conduct a controlled experiment akin to those used in medicine or drug testing. Researchers randomly assign each person to either get the treatment or to not get the treatment.

Unfortunately, the preschool pushers have no experimental evidence that the pre-K programs they promote have a significant, long-term, positive effect.

That's why they rely so heavily on the few pieces of experimental evidence from programs that look nothing like those in Oklahoma, Georgia, or any other state that has adopted or is considering a pre-K program.

Preschool activists kneel before a holy trinity of early-intervention programs that supposedly prove preschool is our educational, nay, our societal savior: the Perry Preschool Project, Carolina Abecedarian Project, and Chicago Child-Parent Centers Program.

Unfortunately, these programs don't come even close to proving what many preschool activists like to pretend they prove-the long-term effectiveness, let alone cost-effectiveness, of current large-scale preschool programs.

These programs were all small-scale, intensive, and targeted at the most disadvantaged children. On the Cato Institute blog (see "Pre-K Pushers Peddling Patent Prevarications" at www.cato-at-liberty.org) I highlight some of the other problems with using these as evidence in support of government preschool programs.

Unfortunately, the preschool evangelists will not shrivel before arguments or facts, for they believe. Their faith in preschool is strong and pure.

Just because the short-term gains for low-income students don't last doesn't mean they can't last. If we can just make all preschools high-quality, and then make all elementary schools high-quality, and then make all high-schools high-quality, and then make all parents high-quality . . . then preschool might sustain something other than negligible improvements.

Perhaps, but almost certainly not.

More likely, if we had all high-quality schools and parents we'd once again find that whether a child learns her letters at 4 instead of 5 doesn't make one flea-hair's bit of difference by the time she (hopefully) graduates high school.

Finland should give the preschool activists pause. It doesn't, but it should.

Children don't begin formal schooling until around 7. At first, no surprise, they don't score as well as many countries who park their kids in classrooms at age 3 or 4. By high school, however, Finland's students are at the top of the pack internationally, and far outperform the laggard United States.

So why this national obsession with preschool? Is it to take the blame off of our ossified government K-12 system? More money for the teachers unions?

Perhaps it provides hope to progressives who place their faith in the power of government but have witnessed only an unyielding failure to sustain effective and meaningful reform in the government K-12 school system.

Perhaps preschool offers a distraction from the despair and fatalism fostered by so obvious a failure of the public sector. A crusade to invigorate the faithful.

Preschool is not our educational salvation, and "reform" of a moribund government K-12 system is a fool's errand.

The most certain way to improve academic performance and life outcomes for all students in this country, rich and poor, is to expand educational freedom.

Adam Schaeffer (Ph.D., University of Virginia) is a policy analyst at the Cato Institute.