  Background: Natural or quasi experiments are appealing for public health
research because they enable the evaluation of events or interventions that are
difficult or impossible to manipulate experimentally, such as many policy and
health system reforms. However, there remains ambiguity in the literature about
their definition and how they differ from randomised controlled experiments and
from other observational designs.
  Methods: We conceptualise natural experiments in in the context of public
health evaluations, align the study design to the Target Trial Framework, and
provide recommendation for improvement of their design and reporting.
  Results: Natural experiment studies combine features of experiments and
non-experiments. They differ from RCTs in that exposure allocation is not
controlled by researchers while they differ from other observational designs in
that they evaluate the impact of event or exposure changes. As a result they
are, in theory, less susceptible to bias than other observational study
designs. Importantly, the strength of causal inferences relies on the
plausibility that the exposure allocation can be considered "as-if randomised".
The target trial framework provides a systematic basis for assessing the
plausibility of such claims, and enables a structured method for assessing
other design elements.
  Conclusions: Natural experiment studies should be considered a distinct study
design rather than a set of tools for analyses of non-randomised interventions.
Alignment of natural experiments to the Target Trial framework will clarify the
strength of evidence underpinning claims about the effectiveness of public
health interventions.
