Grades given by teachers in the course of the 6th year of Swiss region Jura’s primary schools are routinely «adjusted» on the basis of standardized tests. These corrections were the object of the presented study. In spite of the many variables in the context that remained uncontrolled, the grading standards of each teacher could be shown to remain stable over time, while differing from one subject matter to the other. A simple correction of each teacher’s «personal equation» is proposed, as a means to keep these sources of error in check.