Recent advances in automatic classification

Traditional classification methods are divided into two broad types: hierarchical methods and non‐hierarchical methods in which the number of classes has to be fixed in advance. Both methods can handle both quantitative and qualitative data. A third type finds a partition optimizing a linear classification criterion (e.g. the Condorcet criterion) in which the number of classes does not have to be fixed in advance, but the data must be qualitative. A recent generalization, the ‘S theory’ can handle simultaneously both quantitative and qualitative data, and both linear and non‐linear classification criteria (in the space of paired comparisons of elements). With this ‘S theory’ the partition is obtained in order n (in terms of memory space and elementary operations), n being the number of elements to classify.