The cell biologist's understanding might focus on the mechanisms by which respiration occurs in the cell; the physician's understanding might focus on human respiratory disorders and physical and pathological causes.

An ordinary citizen's understanding of respiration will be much less sophisticated

Eliciting and analyzing explanations are useful ways of assessing science achievement.

than that of a practicing scientist. However, even the citizen's understanding will have different perspectives, reflecting differences in experience and exposure to science.

Legitimate differences in perspectives and sophistication of understanding also will be evident in each student's scientific understanding of the natural world. A challenge to teachers and others responsible for assessing understanding is to decide how such variability is translated into judgments about the degree to which individual students or groups of them understand the natural world. The example that follows illustrates how explanations of the natural world can be a rich source of information about how students understand it.

Because explanation is central to the scientific enterprise, eliciting and analyzing explanations are useful ways of assessing science achievement. The example illustrates how thoughtfully designed assessment exercises requiring explanations provide students with the opportunity to demonstrate the full range of their scientific understanding. Exercises of this sort are not designed to learn whether a student knows a particular fact or concept, but rather to tap the depth and breadth of the student's understanding. Exercises of this sort are difficult to design and are a challenge to score. The example that follows illustrates these challenges.

THE PROMPT. The assessment task begins with a prompt that includes a description of the task and directions. The prompt reads

Some moist soil is placed inside a clear glass jar. A healthy green plant is planted in the soil. The cover is screwed on tightly. The jar is located in a window where it receives sunlight. Its temperature is maintained between 60° and 80°F. How long do you predict the plant will live? Write a justification supporting your prediction. Use relevant ideas from the life, physical, and earth sciences to make a prediction and justification. If you are unsure of a prediction, your justification should state that, and tell what information you would need to make a better prediction. You should know that there is not a single correct prediction.

[See Teaching Standard B]

Many attributes make the ''plant in a jar" a good exercise for assessing understanding. The situation, a plant in a closed jar, can be described to students verbally, with a diagram, or with the actual materials, thus eliminating reading as a barrier to a student response. The situation can be understood by students of all ages, minimizing students' prior knowledge of the situation as a factor in ability to respond. The explanation for the prediction can be developed at many different levels of complexity, it can be qualitative or quantitative. It can be based on experience or theory, and it uses ideas from the physical, life, and earth sciences, as well as cross-disciplinary ideas, thus allowing students to demonstrate the full range of

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