that is possible with instruction both in science content and process. Those weaknesses in the knowledge base are reflected in the K-12 science curriculum.

Analyses of TIMSS science achievement results (Schmidt, 2001; Valverde and Schmidt, 1997) as well as research conducted by other investigators show that in contrast to other countries, elementary and middle school science in the United States emphasizes broad coverage of diverse topics over conceptual development and depth of understanding. For example, eighth grade textbooks in the United States cover an average of more than 65 science topics, in stark contrast to the 25 topics typical of other TIMSS countries. “U.S. eighth-grade science textbooks were 700 or more pages long, hardbound, and resembled encyclopedia volumes. By contrast, many other countries’ textbooks were paperbacks with less than 200 pages” (Valverde and Schmidt, 1997:3). The more recent TIMSS-R follow-up study concluded that the comparatively poor performance of U.S. eighth graders is related to a middle school curriculum that is not coherent and is not as demanding as that found in other countries studied. “We have learned from TIMSS that what is in the curriculum is what children learn” (Schmidt, 2001:1).

Commercially published textbooks are the predominant instructional materials used in science (Weiss et al., 2002). In grades K-4, textbooks are used 65 percent of the time; this increases to 85 percent of the time in grades 5-8, and 96 percent of the time in grades 9-12. Most of these textbooks are seriously flawed. A team at the AAAS reviewed widely used textbooks in middle and high school science and ranked them on a number of criteria, among them the extent to which the major concepts were communicated clearly and students’ preconceptions were addressed. All of the middle school textbooks and most of the high school textbooks were rated poor (Roseman et al., 1999). On the critical dimension of supporting conceptual change, widely used science textbooks at both the middle school and high school level have been judged poor by the AAAS team (Roseman et al., 1999).

In recent years, researcher-practitioner collaborations have begun to generate more systematic approaches to science education. These instructional efforts span several grades and are, in a sense, hypotheses about what children can learn and do at different grade levels. The commitments and design trade-offs

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