Where does interaction collide?

Newton's 1687 Philosophiæ Naturalis Principia Mathematica set out that bodies act on one another with mutual force, gravitation drawing every mass toward every other and every action met by an equal and opposite reaction, and this is the origin of the physical idea that particles influence each other. Modern physics groups these influences into the fundamental interactions, gravitational, electromagnetic, strong, and weak, and Weinberg's 1967 A Model of Leptons unified two of them into a single electroweak account. Here the referent is a force in nature that changes how matter and energy move, measured in the laboratory and the sky, and it is not a reciprocal action between people or a statistical effect of one factor upon another.[1][2]

Unit

A force between two bodies or fields, given in newtons or as a coupling strength

Recovery means

Not applicable

Measured by

Inferred from how bodies accelerate or scatter, and from the rates of decays and reactions recorded in detectors

Simmel's 1908 Soziologie made reciprocal action between people, which he called Wechselwirkung, the basic subject matter of the discipline, arguing that society exists in the countless exchanges through which individuals affect one another. Mead's 1934 Mind, Self and Society traced how the self and shared meaning arise in this exchange, and Blumer's 1969 Symbolic Interactionism named the resulting school and held that people act toward things on the basis of meanings worked out between them. Here the referent is the meeting of persons who interpret and respond to each other, studied through observation and conversation, and it is not a force between particles or the joint effect of two factors in an experiment.[3][4][5]

Unit

A single episode of people acting toward and interpreting one another

Recovery means

Not applicable

Measured by

Studied through direct observation, recorded conversation, and coding of what people do and say together

Fisher and Mackenzie's 1923 Studies in Crop Variation II introduced the word interaction to describe a joint effect that plain averaging misses: the response to one treatment differed depending on the potato variety it was applied to, so the two factors could not simply be added together. Fisher's 1935 The Design of Experiments then built the factorial designs and analysis of variance that isolate such effects and test them. Here the referent is a statistical quantity, the part of an outcome that appears only when the levels of two or more factors are combined, estimated from data, and it is not a physical force or a social encounter.[6][7]

Unit

The extra part of an outcome present only when factor levels are combined

Recovery means

Not applicable

Measured by

Estimated as an interaction term in analysis of variance or regression and tested for significance