Editing Experiment (section)

=== Natural experiments ===
{{main|Natural experiment}}
The term "experiment" usually implies a controlled experiment, but sometimes controlled experiments are prohibitively difficult, impossible, unethical or illegal.  In this case researchers resort to natural experiments or [[quasi-experiments]].<ref>{{harvnb|Dunning|2012}}</ref>  Natural experiments rely solely on observations of the variables of the [[system]] under study, rather than manipulation of just one or a few variables as occurs in controlled experiments.  To the degree possible, they attempt to collect data for the system in such a way that contribution from all variables can be determined, and where the effects of variation in certain variables remain approximately constant so that the effects of other variables can be discerned.  The degree to which this is possible depends on the observed [[correlation]] between [[explanatory variables]] in the observed data.  When these variables are ''not'' well correlated, natural experiments can approach the power of controlled experiments.  Usually, however, there is some correlation between these variables, which reduces the reliability of natural experiments relative to what could be concluded if a controlled experiment were performed.  Also, because natural experiments usually take place in uncontrolled environments, variables from undetected sources are neither measured nor held constant, and these may produce illusory correlations in variables under study.{{cn|date=April 2025}}

Much research in several [[science]] disciplines, including [[economics]], [[human geography]], [[archaeology]], [[sociology]], [[cultural anthropology]], [[geology]], [[paleontology]], [[ecology]], [[meteorology]], and [[astronomy]], relies on quasi-experiments.  For example, in astronomy it is clearly impossible, when testing the hypothesis "Stars are collapsed clouds of hydrogen", to start out with a giant cloud of hydrogen, and then perform the experiment of waiting a few billion years for it to form a star. However, by observing various clouds of hydrogen in various states of collapse, and other implications of the hypothesis (for example, the presence of various spectral emissions from the light of stars), we can collect data we require to support the hypothesis.  An early example of this type of experiment was the first verification in the 17th century that light does not travel from place to place instantaneously, but instead has a measurable speed. Observation of the appearance of the moons of Jupiter were slightly delayed when Jupiter was farther from Earth, as opposed to when Jupiter was closer to Earth; and this phenomenon was used to demonstrate that the difference in the time of appearance of the moons was consistent with a measurable speed.<ref>{{Cite web|url=https://www.amnh.org/learn-teach/curriculum-collections/cosmic-horizons-book/ole-roemer-speed-of-light|title=Ole Roemer Profile: First to Measure the Speed of Light &#124; AMNH}}</ref>