Eleanor Finch
Natural experiments are a type of empirical or observational study in which the control and experimental variables are not manipulated by researchers but are influenced by nature or factors outside the researchers' control. They are employed when controlled experimentation is difficult to implement, unethical, or not feasible. Natural experiments are useful in fields such as epidemiology and economics, where they provide a rare opportunity to study complex issues that are challenging to test in controlled settings. While natural experiments can suggest causal relationships, they do not unequivocally prove them, and researchers must carefully analyse and interpret the results while considering potential confounding factors and limitations.
| Characteristics | Values |
|---|---|
| Definition | A natural experiment is a study in which individuals or clusters of individuals are exposed to experimental and control conditions that are determined by nature or by other factors outside the control of investigators. |
| Use Cases | Natural experiments are employed when controlled experimentation is difficult to implement or unethical. |
| Examples | The 1854 Broad Street cholera outbreak in London, the smoking ban in Helena, Montana, and the Brazilian government's audits of local mayors are all examples of natural experiments. |
| Advantages | Natural experiments provide a rare testing ground for researchers, allowing them to study complex issues that are challenging to test in controlled settings. |
| Limitations | The inability to control variables and the presence of unmeasured confounding factors can make it difficult to draw firm conclusions and unequivocally prove causation. |
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What You'll Learn
Natural experiments are observational studies
Natural experiments are also used in economics to study otherwise challenging topics. For example, researchers might be interested in whether voters punish corrupt politicians. When the Brazilian government randomly selected towns to audit mayors for corruption, researchers could compare the reelection chances of audited mayors with those who were not.
Another example of a natural experiment is when a smoking ban was implemented in Helena, Montana, for six months. Investigators observed a 40% drop in heart attacks during this period. This study design is called a case-crossover experiment, where exposure is removed and then returned.
While natural experiments can suggest causal relationships, they do not unequivocally prove them like controlled studies. They are useful when there is a clearly defined exposure involving a well-defined subpopulation, allowing changes in outcomes to be attributed to the exposure.
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They are useful when controlled experiments are difficult or unethical
Natural experiments are a useful alternative when controlled experiments are difficult or unethical to conduct. They are observational studies where the control and experimental variables are not manipulated by researchers but are influenced by factors outside their control. This makes them particularly useful in fields like epidemiology, economics, and the social sciences, where controlled experiments involving human subjects may be challenging or unethical.
For example, in the field of epidemiology, it would be unethical to intentionally expose individuals to varying degrees of ionizing radiation to study its health effects. However, researchers can use natural experiments, such as studying the health impact on people living near Hiroshima at the time of the atomic blast, to gain valuable insights without compromising ethical standards.
Similarly, in economics, natural experiments allow researchers to study complex issues that are challenging to test in controlled settings. For instance, investigating the "return on investment" of higher education in adults or the impact of a smoking ban on heart attack rates in Helena, Montana, would be difficult to replicate in a controlled experiment due to the involvement of large populations and ethical considerations.
Natural experiments can also be useful in the social sciences, where controlled experiments may be challenging or expensive to implement. For example, studying the impact of housing policies on health outcomes would be difficult to conduct experimentally due to ethical concerns. However, a natural experiment could take advantage of a policy change, such as a lottery for subsidized mortgages, to compare the health outcomes of individuals who moved to better housing with those who remained in substandard housing.
While natural experiments have limitations in proving causation due to the lack of randomization and potential confounding variables, they provide valuable insights and are often the only feasible approach for studying certain research questions.
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They can suggest causal relationships but don't prove them
Natural experiments are quasi-experiments that are employed as study designs when controlled experimentation is extremely difficult to implement or unethical. They are most useful when there is a clearly defined exposure involving a well-defined subpopulation and the absence of exposure in a similar subpopulation, such that changes in outcomes may be plausibly attributed to the exposure.
Natural experiments are observational studies where the control and experimental variables are not manipulated by researchers but are influenced by nature or factors outside the researchers' control. The process governing the exposures resembles random assignment, and this allows for causal inference. However, the lack of random assignment means that multiple threats to causal inference, such as attrition, history, testing, regression, instrumentation, and maturation, may influence the observed study outcomes.
Natural experiments can suggest causal relationships but do not prove them. This is a major limitation of natural experiments in inferring causation. While they can provide an important inferential method for researchers to gather information, especially in fields like epidemiology, economics, and the social sciences, they are not sufficient to unequivocally prove causation.
For example, in a well-known natural experiment in Helena, Montana, a smoking ban was implemented in all public spaces for a six-month period. Investigators observed a 40%-60% drop in heart attacks during this time. However, this study does not prove that the smoking ban caused the decrease in heart attacks, as there may be other factors at play, such as personal preferences or reverse causality.
Another example is a natural experiment conducted in Brazil, where researchers wanted to test whether voters punish corrupt politicians. They noticed that the federal government randomly selected a group of towns to be audited for corruption, providing an opportunity to study the impact of audits on the reelection chances of mayors. While this natural experiment can suggest a causal relationship between audits and reelection chances, it does not prove it, as there may be other variables at play that were not controlled for in the experiment.
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They are quasi-experiments
Natural experiments are quasi-experiments, meaning they resemble true, randomized experiments but lack random assignment. In other words, they are observational studies where the control and experimental variables are not manipulated by researchers but are instead influenced by factors outside their control.
Natural experiments are often employed when controlled experimentation is difficult to implement, too expensive, or unethical. For example, in epidemiology, it would be unethical to randomize people to variable housing conditions to study the effect of poor housing on health. However, if a housing policy change enabled some people to move to more desirable housing while leaving others in substandard housing, researchers could use that policy change to study the effect of housing changes on health outcomes.
Similarly, in economics, natural experiments allow researchers to study otherwise challenging topics. For instance, researchers might evaluate the "return on investment" of higher education in US adults, which would be difficult to test in a controlled experiment. Nuclear weapons testing is another example of a natural experiment, as it would be unethical to intentionally release radioactive isotopes into the atmosphere to study their effects on biological tissues.
Game shows can also be considered natural experiments since the context arises without the interference of scientists. For example, game shows have been used to study economic behavior, such as decision-making under risk and cooperative behavior.
While natural experiments can suggest causal relationships, they do not unequivocally prove them like controlled studies. This is because the lack of random assignment introduces multiple threats to causal inference, such as attrition, history, testing, regression, instrumentation, and maturation. Therefore, natural experiments must be carefully analyzed as quasi-experiments, and researchers should consider using methods like instrumental variables to control for confounding and measurement error.
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They are employed in epidemiology and economics
Natural experiments are often used in epidemiology and economics when controlled experimentation is difficult to implement, expensive, or unethical.
In epidemiology, natural experiments can be used to evaluate the health impact of certain events or conditions. For example, the 1854 Broad Street cholera outbreak in London was a natural experiment that helped physician John Snow identify the source of the outbreak as the nearest public water pump. More recently, researchers have been able to study the health impact of exposure to ionizing radiation in people living near Hiroshima at the time of the atomic blast. Another example is the smoking ban enacted in Helena, Montana, for six months, which resulted in a 40% drop in heart attacks.
In economics, natural experiments can be used to study the impact of changes in laws, policies, or practices on a defined population. For instance, researchers have used natural experiments to investigate the economic return on the amount of schooling in US adults and the impact of military service on lifetime earnings. One notable example is a natural experiment conducted in Brazil, where researchers examined the impact of corruption audits on the reelection chances of mayors.
While natural experiments provide valuable insights, they have limitations. The lack of randomization and control in natural experiments means that researchers must be cautious when interpreting results and making causal inferences.
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Frequently asked questions
A natural experiment is a study where individuals or groups are exposed to experimental and control conditions that are determined by factors outside the investigator's control. Natural experiments are observational studies and are useful when controlled experimentation is difficult to implement or unethical.
In natural experiments, researchers observe without manipulating variables, whereas in controlled experiments, researchers artificially manipulate variables. Natural experiments suggest causal relationships but do not prove them unequivocally like controlled studies.
Some examples of natural experiments include the 1854 Broad Street cholera outbreak in London, the smoking ban in Helena, Montana, and the Brazilian government's audits of local mayors for corruption.
Natural experiments are conducted when controlled experimentation is impractical, expensive, or unethical. They provide a rare opportunity to study complex issues and gather information that may not be obtainable through traditional controlled experiments.
The major limitation of natural experiments is the inability to control variables, which can make it difficult to draw firm conclusions and prove causation unequivocally. Natural experiments also face threats to causal inference, such as attrition, history, testing, and maturation.
