Usually a statement asserting that there is no difference or no association between variables. The null hypothesis is a tool that makes it possible to use certain statistical tests to figure out if another hypothesis of interest is likely to be accurate or not.

Of the physical universe. Natural entities include all the components of the physical universe around us like atoms, plants, ecosystems, people, societies, and galaxies, as well as the physical forces at work on those things. To learn more, visit our side trip What’s natural?

All the components of the physical universe — atoms, plants, ecosystems, people, societies, galaxies, etc., as well as the natural forces at work on those things. Elements of the natural world (as opposed to the supernatural) can be investigated by science. To learn more, visit our side trip What’s natural?

A scientific test that mimics the design of an experiment, but that involves phenomena not controllable by the investigator (e.g., planetary movement, tectonic action). In a natural experiment, the researcher takes advantage of a pre-existing situation that happens to provide a test of a particular idea. To learn more, visit Tactics for testing ideas.