word “theory” as an idea or conjecture rather than as a complex explanation supported by evidence. It does not imply that a child’s theory is a scientific theory in the sense defined above. However, a conceptual framework takes the place of a scientific theory in the way that the child uses it to process information and to view and interact with external events; hence the interplay between instruction and a child’s conception of the world is an important issue for the teaching of science.

The second alternative meaning 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 have no broader conceptual scope. Popular usage also confuses the ideas of scientific fact and a scientific theory, which we distinguish by example in the discussion below.

Data and Evidence

A datum 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 here includes indirect observation, which uses inference from well-understood science, as well as direct sensory observations. Thus the assertion that a particular skeleton comes from an animal that lived during a particular geological period is based on acceptance of the body of knowledge that led to the widely accepted techniques used to date the bones, techniques that are themselves the products of prior scientific study. “Observations” in the research laboratory, particularly observations of events and phenomena whose duration or size is inaccessible to the unaided human perceptual system, often include a substantial chain of such inferences. In the elementary and middle school classroom, observation usually involves fewer inferences. 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.

Some use the term “scientific claim” for a well-established property, correlation, or occurrence, directly based on well-validated observation or measurement. When a scientific claim is demonstrated to occur forever and always in any context, scientists will refer to the claim as a fact (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

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