In One Sentence What Once Took Four LONG Parts

Once Again, D-Squared Explains It All:

[T]he Big Mac Index can plausibly claim to be the major methodological forerunner of Freakonomics, as it combines the two methodological techniques of choosing “quirky” instruments more valuable for their amusement value than their validity, and not checking anything to see whether it’s economically meaningful.

I’m not certain that “economically” is strictly necessary there.