Unmeasurable concepts like intelligence and ideology get measured via observable proxy indicators.

Evidence

The central problem in the history of psychology is that of “unobserved” or “latent” variables. Many psychological concepts are not directly measurable, e.g., “intelligence.” One tries to get at them via questionnaires, surveys, and tests. The answers to these questionnaires then become the indicators of the latent variables. These techniques are widely used in political science in order to construct measures of ideology for members of Congress or Supreme Court justices. Votes in Congress become the indicators and left-right measures of ideology are constructed using them. — Gary Goertz, Social Science Concepts and Measurement, 315

Categorization

• DEFINE: Defines latent variable measurement: unobserved concepts operationalized through observable responses, votes, or test items that serve as indicators.
• CANDY: American judges’ and legislators’ ideology scores are computed from nothing more than how they vote — a tidy, repeatable fact for a politics-curious conversation.