We wish to develop descriptions of students’ knowledge and practice that will ultimately include all four strands of scientific proficiency (see Box 2-1) and that recognize the complex organization of meaningful scientific knowledge and practice. Furthermore, we would like to describe children’s knowledge and practice in ways that help us to see the continuities—and the discontinuities—between the reasoning of children of different ages. Inevitably, these descriptions must fail in some way; no organizational scheme can fully capture the organization of a child’s knowledge or its connections with her practices, with systems and phenomena in the material world, and with developmental changes over time. The various approaches to describing core ideas and strands in children’s reasoning discussed in this book represent various compromises that emphasize some aspects of the organization of their reasoning while obscuring others.

In addition to describing children’s knowledge and practice at a given age, learning progressions aspire to describe how that knowledge and practice could change over time, with successive understandings representing an achievable advance from earlier ones. This presents multiple challenges. We wish to describe both continuities and discontinuities in children’s thinking, as well as successional trends over time. The choices we make will inevitably emphasize some of those continuities and discontinuities while obscuring others. In addition, each phase must represent an achievable advance from the one before. The strongest evidence for a suggested advance comes in the form of teaching experiments that demonstrate how students can move from one set of understandings to the next or longitudinal studies showing systematic progressions in students’ understanding. When this kind of empirical evidence is not available, we can suggest stages that represent reasonable advances across all four strands of scientific proficiency.

Finally, no single learning progression will be ideal for all children, since they have different instructional histories, bring different personal and cultural resources to the process of learning science, and learn in different social and material environments. The best learning progressions are those that make effective use of the resources available to different children and in different environments. This is the challenge that we are farthest from responding to effectively with the current research base.

INITIAL WORK ON LEARNING PROGRESSIONS

What might such long-term learning progressions look like? Recently, two design teams that were composed of scientists, science educators, and experts on children’s learning were asked to use existing research to construct possible learning progressions for two important theories in natural science: the atomic-molecular theory (Smith et al., 2004) and evolution (Lehrer, Catley, and Reiser, 2004). These two theories are unquestionably core ideas



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