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Fred Hebert
@ferd.ca

Although there have been changes in cognition and systems theory, I wanted to bring up some stuff Rasmussen published in the 80s that was really elegant. I'll use 3 diagrams he published, covering the ideas of abstraction hierarchies and how people operate systems when troubleshooting them.

6 reposts13 likes

Fred Hebert
@ferd.ca

The first one is the abstraction hierarchy from Coping With Complexity (1981). This shows how requirements and design tends to go top-down based on goals that must be met by the system. However, signals of problems tend to go bottom-up, from a local issue showing up as disrupted functionality.

Fred Hebert
@ferd.ca

The 2nd one (same paper) shows aggregation, where a nuclear power plant can be described as a whole conceptual system (top), as functional sub-units with material elements (middle), or as granular units that get closer to the material world (bottom).

Fred Hebert
@ferd.ca

Rasmussen maps these as a space across two dimensions: abstraction (purpose to physical) and decomposition (whole-system to individual components). Each component can have a physical set of properties, but also a high level purpose it is connected to; so do aggregated portions of a system.

Fred Hebert
@ferd.ca

He then describes how workers do their work by skipping around abstraction layers, going up and down the hierarchies based on both purposes (high-level goals and functions) and reasons (bottom-up signals) but also across various aggregations (whole systems vs. components).