Not all things can be measured practically, and testing of politically-loaded topics (including most laws that have serious opposition) would be inherently biased. Also, as mentioned upthread, many laws would have no effect or drastically different effect when true at only the local level from the effect if national; for example, dry counties (counties with local prohibition) do not resemble the national-scale effects of Prohibition.
Not all things can be measured practically, and testing of politically-loaded topics (including most laws that have serious opposition) would be inherently biased. Also, as mentioned upthread, many laws would have no effect or drastically different effect when true at only the local level from the effect if national; for example, dry counties (counties with local prohibition) do not resemble the national-scale effects of Prohibition.