Is an N² chart the same as a design structure matrix?

They are close relatives: both are square matrices of elements and the dependencies between them. Design structure matrices are more often used to analyze and reorder dependencies, and their row and column conventions vary, so check which one a chart uses.

What does an empty row mean?

That the element on that row sends nothing to the others on the chart. It may be a real feature of the design or an interface missing from the model, and either is worth checking.

Glossary Modeling foundations

What is an N² chart?

An N² chart is a square matrix with a system's parts on the diagonal and their interfaces in the cells between, showing what each part sends and receives.

Also called N2 chart, N-squared chart, N² diagram.

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N² chart

An N² chart is a square matrix that shows the interfaces among N elements of a system, which gives it N² cells. The elements sit on the diagonal. By the usual convention, the cell in row A, column B holds what A sends to B, so a row lists an element's outputs and a column its inputs. It is used for both functional and physical interfaces.

Why it matters

Laid out this way, gaps and patterns are easy to see: an empty row or column marks an element that sends or receives nothing, a cell filled on one side of the diagonal only shows a one-way interface, and dense clusters suggest elements that belong together.

How it goes wrong

Mixing levels of the tree on one chart, so a subsystem sits beside a single part
Reading an empty cell as no interface, when it may only mean none has been modeled yet
Leaving out the interfaces that cross the boundary of the elements on the chart

In Cairn

The Architecture lens has an N² chart for the selected node: its parts on the diagonal and, in each cell, the interfaces from the row's part to the column's, including those that run between deeper parts. When interfaces cross the node's boundary, an Outside row and column show them. The chart is drawn from the same model as the Architecture diagram and the interface list, so the three always agree, and opening an empty cell between two parts starts a new interface.

Questions

Is an N² chart the same as a design structure matrix?: They are close relatives: both are square matrices of elements and the dependencies between them. Design structure matrices are more often used to analyze and reorder dependencies, and their row and column conventions vary, so check which one a chart uses.
What does an empty row mean?: That the element on that row sends nothing to the others on the chart. It may be a real feature of the design or an interface missing from the model, and either is worth checking.

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