Where does cost collide?

In economics, cost is not the money paid out but the value of the best opportunity given up when a scarce resource is committed to one use rather than another. Green's 1894 Pain-Cost and Opportunity-Cost in the Quarterly Journal of Economics drew this distinction, arguing that the real cost of an action is the worth of what its resources could otherwise have produced. The referent is a forgone alternative, a comparison between what was chosen and what was surrendered, and it has nothing to do with a number minimized by a learning algorithm, a loss of offspring, or the felt effort of thinking.[1]

Unit

One choice's cost, valued as the best forgone alternative

Recovery means

Not applicable

Measured by

Estimated by valuing the next-best use of the same resource, in money or utility

In machine learning and optimization, a cost function, also called a loss or objective function, is a single number that scores how far a model's outputs fall from a target, and training proceeds by adjusting parameters to make that number as small as possible. Rumelhart, Hinton, and Williams's 1986 Learning representations by back-propagating errors in Nature showed how the gradient of such an error measure can be propagated backward through a network to update its weights. The referent is a mathematical quantity defined over a model's parameters and reduced by calculation, and it is not a forgone opportunity, a loss of fitness, or an unpleasant feeling of effort.[2]

Unit

A scalar error summed or averaged over training examples

Recovery means

Not applicable

Measured by

Computed from model outputs and targets, and reduced by gradient descent

In evolutionary biology, a cost is a decrement in survival or future reproduction that an organism suffers as the price of some benefit, so that any trait or act is shaped by the balance between what it gains and what it costs in fitness terms. Williams's 1966 Natural Selection, the Costs of Reproduction, and a Refinement of Lack's Principle in The American Naturalist made the cost of reproduction central, arguing that effort spent on current offspring reduces the parent's later survival and breeding. The referent is a loss counted in the currency of fitness, survival and offspring, and it is unrelated to a forgone market opportunity, a number inside an optimizer, or the effort of concentrating.[3]

Unit

A fitness loss, counted in survival or future offspring forgone

Recovery means

Not applicable

Measured by

Inferred from trade-offs between current reproduction and later survival or fecundity

In cognitive neuroscience and neuroeconomics, cost refers to The subjective aversiveness of mental effort, the felt burden of holding attention or overriding a habit, which the brain appears to weigh against expected reward when deciding how hard to work. Shenhav, Botvinick, and Cohen's 2013 The Expected Value of Control in Neuron proposed that the anterior cingulate cortex sets how much cognitive control to deploy by subtracting this effort cost from the anticipated benefit. The referent is an internal, felt penalty on thinking, registered by a nervous system, and it is not a forgone alternative, a minimized training objective, or a loss of offspring.[4]

Unit

The felt penalty on a unit of mental effort, weighed against reward

Recovery means

Not applicable

Measured by

Inferred from choices to avoid demanding tasks and from neural signals of control allocation