A conjectured entity, process, or event that is not observed directly but is assumed to explain an observable phenomenon. It is not merely a summary of the relationships between observable variables but contains surplus meaning over and above such relationships. Typical examples are the echoic store, logogen, primal sketch, and superego. The distinction between hypothetical constructs and intervening variables was first made explicit by the US psychologists Kenneth MacCorquodale (1919–85) and Paul Everett Meehl (1920–2003) in an influential article in the journal Psychological Review in 1948, in which they used the examples from physics of an electron (a hypothetical construct that is assumed to be an entity, although it is not observed directly) and the resistance of a wire (an intervening variable that merely specifies the amperage of current that will be carried by the wire for any given voltage). See also construct, convergent validity, discriminant validity, nomological network.