The term experiment usually implies a controlled experiment but sometimes controlled experiments are prohibitively difficult or impossible. In this case researchers resort to natural experiments also called quasiexperiments. 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.

Much research in several important science disciplines including economics political science geology paleontology ecology meteorology and astronomy relies on quasiexperiments. For example in astronomy it is clearly impossible when testing the hypothesis suns 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 sun. 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 s 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 of light.



Observational study

Observational studies are very much like controlled experiments except that they lack probabilistic equivalency between groups. These types of experiments often arise in the area of medicine where for ethical reasons it is not possible to create a truly controlled group. For example one would not want to deny all forms of treatment for a lifethreatening disease from one group of patients to evaluate the effectiveness of another treatment on a different group of patients. The results of observational studies are considered much less convincing than those of designed experiments as they are much more prone to selection bias. Researchers attempt to compensate for this with complicated statistical methods such as propensity score matching methods see hierarchy of evidence. See also quasiempirical methods