Consistent with one of the key design principles underlying CBAL, we will create a reasonably realistic scenario for each assessment prototype (Bennett, 2010). The literature in problem-based learning (PBL) research posits that engaging students in solving a real-world problem promotes knowledge acquisition, problem solving skills, and motivation through authentic learning tasks (Hmelo & Lin, 2000). Thus PBL, in turn, provides opportunities to measure the integrated application of both conceptual knowledge and science practices. In the scenario context, a driving question will be proposed in a conversational context that sets the goal for the whole task. In this exemplar formative task, students are asked to come up with an answer to the driving question: How can you get pure water to drink out of ocean water? We embedded the driving question in a context where three middle school students and their teacher (Ms. Jessie) are on a school trip at an ocean beach and are having the following conversation:
wiki dialogue.PNG

We selected this driving question because it is situated in a meaningful context for students and therefore should require and promote sense-making during the problem solving process. In addition, it is a complex, open-ended problem that requires students to actively engage in analyzing and refining the problem, explaining, hypothesizing, representing, designing, and conducting investigations to test and revise hypotheses. Therefore, this scenario can provide many opportunities to measure both students’ conceptual understanding and their ability to apply science practices to solve problems. After the scenario is presented, the conversations among the student characters continue to help students refine the driving question into more manageable and testable questions, such as “What is in the pure water and the ocean water?”, “How is pure water different from ocean water?”, “To purify the ocean water, what should be left in and taken out?”, and “What process can help get the pure water back?” Then various activities are designed to help students to respond to those refined questions.
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