Graded categories like World Bank income bands create partial membership across neighboring concepts.

Evidence

Semantically, concepts may lie “near” each other. Often this is the relatively explicit division of the underlying continuum into segments, which is very common. For example, the World Bank defines countries as low, lower-middle, higher-middle, and high income. Another way this can happen is that two concepts share some defining features. As such, it is quite common that a given object or observation can fall under multiple concepts simultaneously. This is partial membership in two or more concepts. — Gary Goertz, Social Science Concepts and Measurement, 475

Categorization

• DEFINE: Introduces ‘partial membership’ — the technical core of fuzzy-set concepts — with a concrete World Bank example.