Defining thirds of schooling years in population studies

When the schooling years are compared between individuals having different birth years, the steep rise in schooling years in the 20th century must be taken into account. The problem has particular importance in large international population studies, such as the WHO MONICA Project and its successor, the MORGAM Project. We present an algorithm that divides the individuals into three groups on the basis of the schooling years while preserving smooth behavior of the cut-points between consecutive birth years. The usage of method is demonstrated with data from Finland, Italy, Lithuania, and Scotland, which have different patterns of the estimated tertiles of schooling years.