What is a Hierarchy?

A hierarchy defines a set of parent-child relationships. Typically, a parent member belongs to a more general concept than its children and the parent member "summarizes" its children. Parent members can further be aggregated as the children of other parents.

A hierarchy can also be viewed as a set of mappings from a set of low-level concepts to higher-level, more general, concepts. Each concept is given a name – its level name. The levels are ordered so that a "more general" level precedes a "less general" level.

Each concept or level consists of a set of values. The level values are called members. Each member has a name and a key where the key is guaranteed to be unique.

As an example, consider a 'Geography' hierarchy that has the concepts of 'Country' and 'City', where 'Country' is more general than City'. That is, the 'Geography' hierarchy has two levels 'Country' and 'City', where the 'Country' level precedes the 'City' level. There are two members; 'US' and 'Germany', who belong to the 'Country' level and four members 'Boston' (whose parent is 'US'), 'Hamburg' (whose parent is 'Germany'), 'Berlin' (whose parent is Germany), and 'Berlin' (whose parent is US), who belong to the 'City' level.

Figure 1. Geographical hierarchy example.

Hierarchies are commonly referred to as categories in Spotfire. That is, hierarchies are generally what you would put on the categorical axis of a visualization. A hierarchy with only one (1) level is represented as a column in Spotfire. A hierarchy with more than one level is represented as a hierarchy in Spotfire.