Abstract: A method for preprocessing reference feature vectors representing patterns in order to form, for each selected pattern class, collections of regions. A hierarchy of possibility regions is formed, wherein all reference feature vectors of a pattern class are contained in each level of the hierarchy of possibility regions associated with the pattern class. This hierarchy is later used to exclude a pattern class from consideration if a feature vector representing an unknown pattern is not contained in some level of its associated hierarchy of possibility regions. A collection of certainty regions is used, wherein no reference feature vector not of a pattern class is contained within any certainty region associated with the pattern class. The certainty regions are later used to classify a feature vector representing an unknown pattern as belonging to a pattern class.