The results suggest that subject-matter expertise; a commitment to students' learning and their lives in a personal, authentic way; and the ability to express oneself in the language used by the student are all determinants of learning in problem-based curricula. The theory of the effective tutor, presented in this article, merges two different perspectives prevalent in the literature. One perspective emphasizes the personal qualities of the tutor: his or her ability to communicate with students in an informal way, coupled with an empathic attitude that enables the tutor to encourage student learning by creating an atmosphere in which open exchange of ideas is facilitated. The other stresses the tutor's subject-matter knowledge as a determinant of learning. The data presented in this article suggest that what is needed, really, is much of both.

Nowadays many schools in higher education implement problem-based learning to foster active learning processes by students. In some schools with a number of years of experience with this approach, phenomena can be observed which indicate signs of wear. The implementation of a large-scale innovation such as problem-based learning (PBL) seems to provoke different activities and attitudes in those actors who are most involved. Students and staff members seem to behave in a way which could be counterproductive to the development of self-directed learning. In the first part of this paper, we briefly describe the cognitive psychological background of PBL. In the second part various adjustments observed in problem-based curricula and their effects on students' learning are analyzed. Arguments are presented about adverse effects on the implementation of this educational innovation. Special attention is given to teachers' concerns. In the third part suggestions are made about ways to revitalize PBL processes as well as suggestions about effecting educational innovations on a more solid basis.

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Two experiments assessed effects of activation of prior knowledge through small-group discussion. Subjects were given a description of natural phenomena and were asked to elaborate on possible explanations for them. In Experiment 1, small groups of subjects were presented with a problem describing the behavior of a blood cell in pure water and in a salt solution. No additional text was studied. The experimental subjects produced more than twice as many propositions about osmosis (i.e. the biological process explaining the blood cell's behavior) as a control group produced. Experiment 2 investigated effects of problem analysis on subsequent text processing for subjects with imprecise prior knowledge (novices) and subjects with precise knowledge (experts). Recall of the text showed considerable facilitative effects of problem analysis. Results are explained in terms of faster accessibility of prior knowledge and better integration of new information into explanatory models that may exist before, or are actively constructed during, problem analysis.

The results indicate that, at least for the curriculum studied, the assumption in the literature that tutors do not necessarily need content knowledge so long as they are skilled in the tutoring process is not entirely justified: the students who were guided by content experts achieved somewhat better and spent more time on self-directed learning. More important, tutoring skill and content knowledge seemed to be necessary and closely related conditions for effective tutoring.