We are all familiar with the notion of a hierarchy, i.e., structures of the following kind:

Figure 2.2

Figure 2.3

There is a range of metaphors to describe hierarchies, including:

  - A hierarchy is like a tree, with one trunk, many branches, and many leaves, though in the diagrams above the trees are upside down with the topmost item being the root

  - Drawing on the notion of a family tree, we describe items in the hierarchy as having a

parent and children.

  - A hierarchy sometimes resembles a pyramid, with a wide flat base and each level narrower than the one below.

A hierarchy is a nested structure of groups and subgroups, with all the children of an item constituting one group.

    Hierarchical grouping is organizing items and groups into hierarchical

    structures.

A coherent hierarchy is one that is well-thought-out and well-presented. The main aspects of coherence for hierarchies are:

1.   Coherent Groups        

The hierarchy’s constituent groups should of course be themselves coherent – i.e., complete, internally distinct, minimal, and ordered.

2.   Abstraction

The main principles of abstraction for hierarchies are captured in this simple diagram:

  - Higher = more general. The ‘higher’ in the hierarchy (closer to the root) the item, the more general or abstract it should be.

Figure 2.4

  - Level Consistency. Items at the same level should be at approximately the same level of abstraction, even across branches.

  - Level Completeness. All appropriate levels of abstraction should be present; there should be no ‘missing rungs’.

3.   Congruence

A hierarchy is fully congruent when similar grouping principles are used throughout the hierarchy. Congruence has two aspects:

  - Horizontal Congruence. Groups at the same level (e.g., B1, B2 and C1, C2) have the same grouping principle.

  - Vertical Congruence. Groups have similar grouping principles to those immediately above and below them.

Figure 2.5

Desirable, not compulsory

Ideally, a hierarchy will satisfy all principles of coherence, but in some circumstances you may want to relax some of the constraints. This is OK as long as you understand clearly what you are doing and why you are doing it.

See also: Topic Grouping