activities, for example, it is important to keep in mind both the development of the students' understandings and skills about the process of investigation and also the aim of developing concept understanding in relation to the phenomena being studied in that investigation. If skills of communication, or the development to reflect on one 's own thinking (metacognition), are aims of the curriculum, then these also have to be in a professional-development agenda.

Implementing effective formative assessment requires that a teacher elicit information about the students' understandings as they approach any particular topic. This is particularly important since a student will likely interpret new material in the framework of her preexisting knowledge and understanding (first main principle from How People Learn, NRC, 1999a). Professional development that will lead to improved assessment must begin with the sensitivity to the need of the teacher to learn how to obtain information about a student's current level of understanding of the subject to be taught and learned.

A teacher influenced by the importance of probing student current knowledge started his teaching of a new science topic with questions designed to elicit the existing understanding. He found that the class knew far more about energy than he had anticipated but lacked a coherent structure in which they could relate their various ideas. He thus abandoned the formal presentations of the whole menu of relevant knowledge that he had emphasized in previous years, and had intended to use again, and attempted instead to help them reorganize their existing understandings. He was able to incorporate student investigations into the unit that helped students' challenge their ideas and apply concepts to everyday events. Overall, the work now took less time than before but was more ambitious in developing understanding of the concepts involved.

As this example demonstrates, teachers must develop and use means to elicit students' existing ideas and understandings. This may be achieved by direct questioning, whether orally, with individuals or in group discussions, or in writing. However, such questioning may be more evocative if it is indirect, if it is about relevant phenomena or situations that are put before students and about which they have to think in order to respond. The responses may then indicate how they interpret the concepts and skills that they possess and choose to bring to bear on the specific problem. For example, the teacher above could ask his students at the outset to try to define mechanical, kinetic or potential energy, or he could provide the students with a