As states' information-collection systems grow more sophisticated, officials are grappling with where to draw the line on how "value added" data on teachers can be used. Since the adoption of the federal No Child Left Behind Act, the task of establishing data systems for tracking students' year-to-year achievement gains has fallen largely to the states. Value-added techniques are one way of dissecting such data. Proponents of the systems say value-added information holds promise for being integrated as one of several factors in human-capital decisions, such as evaluations. Many states are laying the groundwork for such systems by assigning each teacher a unique identification number. Few of those states have set teacher policies that incorporate the data, though--in part, observers say, because value-added systems are a political minefield. States that have progressed on value-added policies--such as Louisiana, which uses the data for teacher-preparation program accountability--typically don't use the systems for individual teacher decisions. Researchers note that technical factors can affect the systems' ability to make reliable determinations of teacher performance, an issue that has raised concerns from the national teachers' unions.