  The term natural experiment is used inconsistently. In one interpretation, it
refers to an experiment where a treatment is randomly assigned by someone other
than the researcher. In another interpretation, it refers to a study in which
there is no controlled random assignment, but treatment is assigned by some
external factor in a way that loosely resembles a randomized experiment---often
described as an "as if random" assignment. In yet another interpretation, it
refers to any non-randomized study that compares a treatment to a control
group, without any specific requirements on how the treatment is assigned. I
introduce an alternative definition that seeks to clarify the integral features
of natural experiments and at the same time distinguish them from randomized
controlled experiments. I define a natural experiment as a research study where
the treatment assignment mechanism (i) is neither designed nor implemented by
the researcher, (ii) is unknown to the researcher, and (iii) is probabilistic
by virtue of depending on an external factor. The main message of this
definition is that the difference between a randomized controlled experiment
and a natural experiment is not a matter of degree, but of essence, and thus
conceptualizing a natural experiment as a research design akin to a randomized
experiment is neither rigorous nor a useful guide to empirical analysis. Using
my alternative definition, I discuss how a natural experiment differs from a
traditional observational study, and offer practical recommendations for
researchers who wish to use natural experiments to study causal effects.
