“In a perfect world, an economist could run a controlled experiment just like a physicist or a biologist does: setting up two samples, randomly manipulating one of them, and measuring the effect. But an economist rarely has the luxury of such pure experimentation... What an economist typically has is a data set with a great many variables, none of them randomly generated, some related and others not. From this jumble, he must determine which factors are correlated and which are not.”
Levitt and Dubner, Freakonomics (2005), page 162.
At the Vernon Smith Experimental Economics Laboratory, we have the luxury of pure experimentation--although
it's far from a "perfect world."