A hierarchy originally meant something like "The sacred rule". It is a ranking of things as to their authority. Hierarchies are foreign to Americans as there is no royalty from which this word hales. Today it is similar to pecking order or business organizational chart.


Most generally it refers to some sort of branching diagram similar to a tree structure.

Systems Engineers employ several types of hierarchies. Two of the more important hierarchies are class and containment.

A containment hierarchy tells what contains what. In manufacturing this is called the bill of materials. Manufacturing resource planning systems use containment hierarchies to order parts. For example; a truck contains an engine and an engine contains a cylinder etc.

Another hierarchy important to a systems engineer is a class hierarchy. A class hierarchy is an abstraction hierarchy. Things at the top are more abstract than things on the bottom.

For example; A truck is a class of systems used to haul material. A pickup truck is a special breed of truck that can haul about half a ton of material around. A Ford pickup truck has a particular style and performance. In a class hierarchy things at the top are more general while things at the bottom are more specialized.

Usually in a containment hierarchy the contained things are smaller than the container while in a class hierarchy the subclasses are often the same size.

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