I favor further experimentation with school vouchers

In Praise of Uncertainty from Marginal Revolution by Tyler Cowen
Writing in the comments, David R. Henderson asks me to list three policy areas where my views are uncertain. Since this blog (or at least this author) has been streaming uncertainty for over four years, this strikes me as an odd request. But perhaps it is useful to have such a list in one place, so here goes:

1. We must address the looming crisis in medical care costs but how? I am uncertain as to how much means-testing Medicare will ease future budgetary pressures. I do favor means-testing, mostly through lack of better ideas, but it is a) notoriously difficult to enforce, b) often unfair (do we measure income or wealth? current or lifetime?) and c) an implicit hike in marginal tax rates. And if you could talk me out of means-testing, I am not sure which recommendation would come next.
2. I favor further experimentation with school vouchers, but to what extent? There are many good school districts that probably would not be improved much if at all, and the resulting political hand-wringing would be costly and also could give vouchers a bad name. Should vouchers be isolated experiments or implemented on a near-universal basis? Near-universal vouchers run the risk of becoming the new middle class entitlement.
3. I don't see a Social Security "crisis" in the numbers, but I do believe we should be fiscally conservative with the program, most of all because of forthcoming Medicare expenditures. Yet this view would be wrong if the growth rate of the economy exceeds the real rate of interest. We could then spend as much on Social Security as we wanted to. (Growth-optimistic conservatives rarely emphasize this conclusion, I might add.) I do not expect such a result, but I give it a probability of about 30 percent.
4. I am uncertain how much the United States should "move first" with costly anti-global warming measures, assuming that China and other nations are not very cooperative.
5. To what extent is the ongoing loss of biodiversity a very serious problem? I suspect in the long run this will prove a more important issue than global warming, but I am not sure. I also don't know what to do about it; property rights and better quotas for fishing is a good idea but that only dents the larger problem.

6. I favor legalizing or decriminalizing many drugs, but I am not sure how far this process can go when so many actual and potential drug customers are under eighteen years of age. Can we really sell crack cocaine in the 7-11, provided there is an ID check for every buyer?
7. I am pro-immigration relative to either current policy or the median voter, but I am uncertain how many immigrants the United States could take in. I'm not just whinging about not knowing where the decimal point goes. More generally, we don't know when the social and political fabric will start to crack in counterproductive fashion.
8. I am highly uncertain about most of the major questions in foreign policy, for a start try Pakistan or the Koreas or nuclear proliferation. Even if you think we shouldn't have gotten involved in the first place, that doesn't mean immediate withdrawal is our best option. And while I know more about economics than foreign policy, I find that the more I learn about a given foreign policy area, the more uncertain I become.
9. Virtually any question in water policy. This is a good, complex area for shaking up policy preconceptions.

That's a lot of uncertainty. I could go on, but that's already most of the major policy issues today. Don't forget this: even if your view is the one "most likely to be right," in absolute terms your view, like mine, is probably wrong relative to the sum of competing views.
In other words, it is hard for me to see why, in these and many other areas, we should be highly certain of the views we hold.
At some point I'll give you my take on "What I Think We Should Be (Nearly) Certain About." But I am not yet sure what should go in that post.