Where does cohesion collide?

The degree to which the elements inside a single software module belong together and serve one well-defined purpose, so that the module does one thing rather than a scattered collection of unrelated tasks. In Structured Design (1974) Stevens, Myers, and Constantine set cohesion opposite coupling and ranked it on a scale from coincidental, where elements are grouped by accident, up to functional, where every element contributes to a single task; good design pushes cohesion high and coupling low. The referent is the internal togetherness of one module of code, and its unit is how strongly that module's parts relate to a shared job. What marks this meaning off from the others is that high cohesion is a design goal an engineer works toward, a judgement about how code is organized, not a force in matter, a bond between people, or a tie between sentences.[1]

Unit

How strongly one module's internal elements serve a single purpose

Recovery means

Not applicable

Measured by

Judged qualitatively on the cohesion scale from coincidental to functional, or by metrics such as lack of cohesion of methods (LCOM)

The attractive force between like molecules of a single substance, the pull that holds a body of matter together and resists its being drawn apart. In the physics of intermolecular and surface forces, treated in Israelachvili's Intermolecular and Surface Forces (2011), cohesion arises from van der Waals and related attractions between identical molecules, and it is what raises the surface tension of a liquid and lets water climb a fine tube. It is set against adhesion, the attraction between unlike materials, so a drop beads up when cohesion beats adhesion and spreads when adhesion wins. The referent is a body of matter and the forces inside it, and the unit is an energy or force per unit area. Here cohesion is a measured fact of nature, neither good nor bad and nothing to design, unlike the software meaning that names a goal for code.[2]

Unit

The attractive force or energy per area holding a substance's molecules together

Recovery means

Not applicable

Measured by

Inferred from surface tension, cohesive energy, or the work needed to separate a material

The total field of forces that act on members to remain in a group, the resultant of everything that makes people want to stay and take part. Festinger defined group cohesiveness in 1950 as exactly this resultant of forces, driven by the attractiveness of the group and its members and by how far the group helps members reach goals they value. Later work made the idea measurable: Carron, Widmeyer, and Brawley's Group Environment Questionnaire (1985) splits cohesion into task and social components at both the individual and group level. The referent is a group and the bond between it and its members, and the unit is the strength of that pull, scored by self-report. This meaning concerns people and their belonging, not code, not molecules, and not the ties within a text; a cohesive team and a cohesive liquid share only the metaphor of holding together.[3][4]

Unit

The strength of the forces binding a member to a group

Recovery means

Not applicable

Measured by

Self-report scales such as the Group Environment Questionnaire, scored on task and social cohesion

The set of surface grammatical and lexical links that connect the sentences of a text to one another, the visible threads that signal that one part depends on another for its interpretation. Halliday and Hasan's Cohesion in English (1976) catalogues these ties, reference, substitution, ellipsis, conjunction, and lexical repetition, and treats cohesion as a property of the text's surface, countable in the links it contains. They keep it distinct from coherence, the deeper meaning that a text hangs together and is about something: a passage can be richly cohesive yet incoherent, or coherent with almost no cohesive markers. The referent is a stretch of discourse and the formal ties within it, and the unit is a cohesive tie between sentences. Unlike group cohesion this pulls sentences together, not people, and unlike the software meaning it describes language as found rather than a goal to design.[5]

Unit

A grammatical or lexical tie linking one sentence to another

Recovery means

Not applicable

Measured by

Counted and classified as cohesive ties: reference, conjunction, ellipsis, substitution, and lexical links