Where does signal collide?

A costly, observable action a job applicant takes to convey a quality the employer cannot directly see, such as acquiring education whose cost is lower for the more able. Spence's 1973 Job Market Signaling showed that such an action works only when it is more expensive for low-quality types, so that it separates them from high-quality types in equilibrium. The referent is a strategic choice made to reveal hidden information, and its meaning rests on the cost that makes it credible, not on any waveform or physical quantity.[1]

Unit

A costly, credible action conveying unobservable quality

Recovery means

Not applicable

Measured by

Modelled rather than scored: a separating equilibrium in which the signal's cost falls as the sender's type rises

A time-varying physical quantity, such as a voltage or a pressure wave, that carries information and must be told apart from noise. In engineering following Shannon's 1948 A Mathematical Theory of Communication, the signal is the message as it travels through a channel, and the task is to represent, transmit, and recover it despite corruption. The referent is a measurable waveform, and what matters is its information content and its fidelity, not any intent behind it or any judgement about it.[2]

Unit

A time-varying quantity carrying information over a channel

Recovery means

Central to the field: recovering the transmitted signal from a noisy channel by detection and estimation

Measured by

Sampled and quantified as a waveform; characterised by bandwidth, power, and signal-to-noise ratio

The stimulus an observer tries to detect against a background of noise, where the word names the present-versus-absent state the observer must decide between. Green and Swets's 1966 Signal Detection Theory and Psychophysics separated an observer's sensitivity, d', which is how far the signal distribution sits from the noise distribution, from their criterion, which is how much evidence they demand before answering present. Here the signal is a perceptual event and the object of study is the decision about it, so the same word points to a judgement rather than to a waveform or a strategic action.[3]

Unit

A to-be-detected event, and the decision separating it from noise

Recovery means

Not applicable

Measured by

Estimated from hit and false-alarm rates as sensitivity (d') and criterion (c or beta)

A display or trait an animal produces to convey information to another, whose reliability is enforced by its cost. Zahavi's 1975 handicap principle argued that a signal stays honest precisely because it is expensive to produce, so that only high-quality individuals can afford the extravagant version. The referent is an evolved trait shaped by selection on both sender and receiver, and its meaning rests on fitness and honesty, which is a different object from a job-market action, a waveform, or a perceptual decision.[4]

Unit

An evolved, costly display conveying quality to a receiver

Recovery means

Not applicable

Measured by

Inferred from the trait's cost and the receiver's response in field, comparative, and experimental studies

The chain of molecular events by which a cell detects an external chemical signal and converts it into an internal response: a signalling molecule binds a receptor, which triggers second messengers that relay and amplify the message to downstream effectors. Sutherland's 1972 Studies on the Mechanism of Hormone Action established the second-messenger concept, showing that a hormone acting at the cell surface raises cyclic AMP inside the cell to produce its effect. The referent is a biochemical relay with no sender choosing to be honest and no cost enforcing credibility, and it is unrelated to a strategic act in a market or an evolved display.[5]

Unit

A cell's conversion of an external signal into a response

Recovery means

The response switches off as second messengers are degraded and receptors are desensitised between stimuli

Measured by

Traced biochemically through receptor binding, second-messenger concentration, and downstream effector activity