Natural experiment

[1][2] In this sense, the difference between a natural experiment and a non-experimental observational study is that the former includes a comparison of conditions that pave the way for causal inference, but the latter does not.

Natural experiments are employed as study designs when controlled experimentation is extremely difficult to implement or unethical, such as in several research areas addressed by epidemiology (like evaluating the health impact of varying degrees of exposure to ionizing radiation in people living near Hiroshima at the time of the atomic blast[3]) and economics (like estimating the economic return on amount of schooling in US adults[4]).

[6][7][8] An aim of a study Angrist and Evans (1998)[9] was to estimate the effect of family size on the labor market outcomes of the mother.

[11] In Helena, Montana a smoking ban was in effect in all public spaces, including bars and restaurants, during the six-month period from June 2002 to December 2002.

The study also noted its own weaknesses which potentially suggest that the inability to control variables in natural experiments can impede investigators from drawing firm conclusions.

'[12] Nuclear weapons testing released large quantities of radioactive isotopes into the atmosphere, some of which could be incorporated into biological tissues.

Several types of observations were made possible (in people born before 1963), such as determination of the rate of replacement for cells in different human tissues.

[13] Using statistical methods developed in econometrics,[14] Angrist capitalized on the approximate random assignment of the Vietnam War draft lottery, and used it as an instrumental variable associated with eligibility (or non-eligibility) for military service.

The effect led the evolutionary biologists L. M. Cook and J. R. G. Turner to conclude that "natural selection is the only credible explanation for the overall decline".