In a field test in May, 1994, we collected nine tapes from each country. We convened a team of six code developers—two from Germany, two from Japan, and two from the United States—to spend the summer watching and discussing the contents of the tapes in order to develop a deep understanding of how teachers construct and implement lessons in each country.

The process was a straightforward one: we would watch a tape, discuss it, and then watch another. As we worked our way through the 27 tapes we began to generate hypotheses about what the key cross-cultural differences might be. These hypotheses formed the basis of codes, i.e., objective procedures that could be used to describe the videotapes quantitatively. We also developed some hypotheses about general scripts that describe the overall process of a lesson, and devised ways to validate these scripts against the video data.

In this way we developed codes to describe a number of dimensions along which the lessons varied, including the kind of mathematics studied, the ways in which lessons were organized, and the kind of thinking students were engaged in during the lesson.