the visible and tangible macroscopic world. Then the domain of phenomena and systems considered is broadened to those that students cannot directly see but that still operate at the scales of human experience. Students then move to exploring or envisioning things that are too small to see or too large to readily imagine, and they are aided by models or specialized tools for measurement and imaging.

This overarching progression informs the grade band endpoints in the framework. Grades K-2 focus on visible phenomena with which students are likely to have some experience in their everyday lives or in the classroom. Grades 3-5 explore macroscopic phenomena more deeply, including modeling processes and systems that are not visible. Grades 6-8 move to microscopic phenomena and introduce atoms, molecules, and cells. Grades 9-12 move to the subatomic level and to the consideration of complex interactions within and among systems at all scales.

Recommendation 8: Whenever possible, the progressions in standards should be informed by existing research on learning and teaching. In cases in which insufficient research is available to inform a progression or in which there is a lack of consensus on the research findings, the progression should be developed on the basis of a reasoned argument about learning and teaching. The sequences described in the framework can be used as guidance.

Because research on these progressions is relatively recent, there is not a robust evidence base about appropriate sequencing for every concept, core idea, or practice identified in the framework. When evidence was available, the committee used it to guide the thinking about the progression in question. When evidence was not available, we made judgments based on the best knowledge available, as supported by existing documents such as the NAEP 2009 Science Framework [15], the College Board Standards for College Success [16], and the AAAS Atlas of Science Literacy [17]. There is also a body of research on the intuitive understanding that children bring to school and on how that intuitive knowledge influences their learning of science [4]; this evidence base should be considered when developing standards.

Each progression described in the framework represents a particular vision of one possible pathway by which students could come to understand a specific core idea. The committee recognizes that there are many possible alternate paths and also that there are interplays among the ideas that here are subdivided into disciplines and component ideas within a discipline. In any case, progressions



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