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Find definitions for the terminology used throughout the Understanding Science pages.

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natural experiment
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.

natural world
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?

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?

null hypothesis
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.

Not influenced by biases, opinions, and/or emotions. Scientists strive to be objective in their reasoning about scientific issues.

To note, record, or attend to a result, occurrence, or phenomenon. Though we typically think of observations as having been made "with our own eyes," in science, observations may be made directly (by seeing, feeling, hearing, tasting, or smelling) or indirectly using tools. To learn more, visit Observation beyond our eyes.

over-arching theory
A term used here to refer to fundamental and particularly broad explanations for many aspects of the natural world. Over-arching theories often help define scientific disciplines and embody the principles that are at the core of our understanding of phenomena in that discipline. Examples include plate tectonics and evolution. To learn more, visit Science at multiple levels.

Principle suggesting that when two explanations fit the observations equally well, a simpler explanation should be preferred over a more complex one. To learn more, visit Competing ideas: Other considerations.

peer review
A method of vetting articles. Articles submitted to a peer-reviewed publication are sent out to several scientists who work in the same field as the paper's author. Those reviewers provide feedback on the article and tell the editor of the publication whether or not they think the study is of high enough quality to be published. To learn more, visit Scrutinizing science: Peer review.

philosophy of science
The study of what science is and how science works. These topics are not ones for which cut-and-dried explanations exist, and the philosophy of science is an area of active debate and investigation. To learn more, visit The philosophy of science.

placebo effect
Phenomenon in which a patient experiences an improvement (or apparent improvement) in a medical condition simply as the result of receiving some sort of treatment — not because of the effectiveness of the treatment itself. Placebo effects are common and can make it difficult to evaluate new medical treatments because even ineffective treatments may appear helpful. To account for the placebo effect, many clinical trials compare a control group receiving a placebo to an experimental group receiving a new treatment. To learn more, visit our side trip Fair tests in the field of medicine.

In medical studies, a treatment, medicine, or therapy given to study participants that is known to have no therapeutic effect on the condition of interest. Placebos are used in clinical trials because they help control an important variable: whether or not the participants know that they are receiving treatment for a condition. With a placebo, neither the participants receiving an experimental treatment and nor those in the control group know whether they are receiving the treatment. To learn more, visit our side trip Fair tests in the field of medicine.

In science, a possible outcome of a scientific test based on logically reasoning about a particular scientific idea (i.e., what we would logically expect to observe if a particular idea were true or false). This website generally uses the term expectation in place of prediction. To learn more, visit our misconception on prediction.

pure science
Research undertaken to build knowledge and understanding, regardless of its potential applications. The boundary between pure and applied science is fuzzy. Research undertaken in the pure pursuit of knowledge often ends up having useful applications, and research begun with an application in mind often ends up informing our understanding of the natural world more broadly.

In reference to the process of science, to repeat a study using methods equivalent to the original's and obtain similar results. Sometimes the term is also applied to situations in which one study's findings are backed up by the results of another study, regardless of the methods employed. Since science aims to uncover the broadly applicable rules by which the universe operates, scientists aim for their studies' findings to be replicable. When a study cannot be replicated, it suggests that our current understanding of the study system or our methods of testing are insufficient. To learn more, visit Copycats in science: The role of replication.

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