Hierarchical classification systems are partly based on expert judgement about pedogenesis, while Numerical systems attempt to be objective classification based on the actual differences between individual soils. These are usually called "ordination" because they attempt to impose an order on the actual data. The general idea is to minimize within-class variance, and maximize between-class variance, according to some objective criterion. The objective information, rather than pre-conceptions about the soils, guides the classification.