Teacher's role and reproducibility of didactical situations

Is it possible to specify teaching situations and to control their effects on students' learning? This general question is essential regarding the scientific character of research on mathematics education. It especially concerns the teacher's role in the class: can she ensure sufficient fidelity to a predefined scenario? This question is of special importance in the case of an experimental scenario the design of which is the result of a collaboration between a researcher and a teacher. This is considered here in the context of two case-studies. These case-studies exhibit two types of factors which tend to hamper fidelity in reproducing a given scenario:- first, constraints on the teacher resulting from the didactical system: time constraints, success of the teaching process;- second, teachers' conceptions about mathematics and learning. These results are considered with reference to prior research on didactical transposition and teacher's beliefs.