Through those tests and the resulting refinement, it takes a form that is a well-established description of, and predictor for, phenomena in a particular domain. A theory is so well established that it is unlikely that new data within that domain will totally discredit it; instead, the theory may be modified and revised to take new evidence into account. There may be domains in which the theory can be applied but has yet to be tested; in those domains the theory is called a working hypothesis. Indeed, the term “hypothesis” is used by scientists for an idea that may contribute important explanations to the development of a scientific theory. Scientists use and test hypotheses in the development and refinement of models and scenarios that collectively serve as tools in the development of a theory.

Outside science, the term “theory” has additional meanings, and these other meanings differ in important ways from the above use of the term. One alternative use comes from everyday language, in which “theory” is often indistinguishable in its use from “guess,” “conjecture,” “speculation,” “prediction,” or even “belief” (e.g., “My theory is that indoor polo will become very popular” or “My theory is that it will rain tomorrow”). Such “theories” are typically very particular and unlike scientific theories have no broader conceptual scope.

A datum—or “data” in plural form—is an observation or measurement recorded for subsequent analysis. The observation or measurement may be of a natural system or of a designed and constructed experimental situation. Observation, even in the elementary and middle school classroom, may be direct or may involve inference or technological assistance. For example, students may begin by conducting unaided observations of natural phenomena and then progress to using simple measurement tools or instruments, such as microscopes.

Evidence is the cumulative body of data or observations of a phenomenon. When the evidence base provides very persistent patterns for a well-established property, correlation, or occurrence, this becomes the basis for a scientific claim. Scientific claims, always based on evidence, may or may not stand the test of time. Some will eventually be shown to be false. Some are demonstrated to occur forever and always in any context, and scientists refer to these claims as factual (e.g., the sun rises in the east). Facts are best seen as evidence and claims of phenomena that come together to develop and refine or to challenge explanations. For example, the fact that earthquakes occur has been long known, but the explanation for the fact that earthquakes occur takes on a different meaning if one adopts plate tectonics as a theoretical framework. The fact that there are different types of earthquakes (shallow and deep focus) helps deepen and expand the explanatory power of the theory of plate tectonics.

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