It should also be falsifiable and admit a counterfactual.
1. The US was within some epsilon of being libertarian;
2. The US became rich;
3. Therefore, the prosperity of the US was a direct consequence of libertarianism.
It's 2. here that I scratch my head at because, uh, the US has through its entire history, even pre-1776, been rich. It wasn't liquid wealth in the financial sense, but the US has had oodles of arable land, navigable waterways[1], natural resources, and favorable geography. It's only within the last 250 years that we've realized wealth in the form of bonkers huge internal markets and (at least, before recently) high levels of immigration.
In fact, going back to 1., if we consider the laissez-faire dimension alone and compare it against the rate of wealth growth the US has sustained at least over my lifetime, the relationship is inverted[2]: we've somehow managed to accumulate more wealth as we've become less laissez-faire. This is a confounding variable that needs to be controlled for before we can draw a causal inference either way.
I'd also argue that broader land policy, education, formal banking institutions, infrastructure, immigration policy, patents and other forms of economic protectionism, and research and development all belong in that causal model since they also tend to be things I look at when analyzing risk and uncertainty.
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0: I'm still unsure if this quantification is even appropriate. Libertarian-ness admits a whole-ass vector space of institutional characteristics, and it isn't obvious to me how the norm would fall down to aggregate all of them as a single scalar for the purposes of making this argument.
1: The fact that pre-Columbian civilizations didn't have the wheel was often a justification for how backwards they were. As it turns out, when you have as many rivers as North America has, getting around without the wheel is no big deal. It's just pre-Columbian YAGNI.
2: Well, kinda. Both "wealth" and "laissez-faire-ness" minimally need operational definitions and some kind of partial order to be comparable like this.