sis on scientific processes can result in instruction about a bundle of topics that are loosely, if at all, related to each other because the development of scientific reasoning does not depend on the treatment of particular topics. Units on weather, electricity and magnetism, and the rain forest follow each other, in an organization most charitably described as modular. Knowledge accumulated in early grades does not build smoothly toward the scientific ideas that will be encountered in high school study and beyond.

In contrast, those who endorse a content approach seek coherence by emphasizing the integrated development of knowledge within scientific disciplines, like biology or chemistry. In practice this emphasis often leads to a focus on concepts and facts—the products of science—with little attention to how that knowledge was generated. In earlier grades, students receive instruction that jumps from earth sciences to physical sciences to biological sciences. The usual result is superficial or fragmentary understanding (Vosniadou and Brewer, 1989; Pfundt and Duit, 1991).

Neither of these views is well aligned with the vision sketched in national science standards (e.g., those from the American Association for the Advancement of Science [AAAS] and the National Research Council). The standards point toward the big ideas and themes that ought to be the goals of science education. For example, AAAS’s report (1991) Science for All Americans proposes that by the time they graduate, students should understand important scientific themes like systems, models, constancy and change, and scale. AAAS points out that these themes “transcend disciplinary boundaries and prove fruitful in explanation, in theory, in observation, and in design” (p. 165). However, there are few illustrations in practice of what it means to understand these ideas deeply, and few guideposts to help teachers navigate the very extensive list of topics that the standards include so that students will arrive at deep understanding of these themes or organizing big ideas. Overall, very little is known about how the material outlined in the standards is actually attainable over the time course of schooling.

The Route: Progression of Understanding

From a very young age, children begin to impose order on the world they observe, generating ideas about why objects float or sink, about what it means to be alive, about why plants



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