Marginal value


A marginal value is
  1. a value that holds true given particular constraints,
  2. the change in a value associated with a specific change in some independent variable, whether it be of that variable or of a dependent variable, or
  3. the ratio of the change of a dependent variable to that of the independent variable.
.
In the case of differentiability, at the limit, a marginal change is a mathematical differential, or the corresponding mathematical derivative.
These uses of the term “marginal” are especially common in economics, and result from conceptualizing constraints as borders or as margins. The sorts of marginal values most common to economic analysis are those associated with unit changes of resources and, in mainstream economics, those associated with infinitesimal changes. Marginal values associated with units are considered because many decisions are made by unit, and marginalism explains unit price in terms of such marginal values. Mainstream economics uses infinitesimal values in much of its analysis for reasons of mathematical tractability.

Quantified conception

Assume a functional relationship

Discrete change

If the value of is discretely changed from to while other independent variables remain unchanged, then the marginal value of the change in is
and the “marginal value” of may refer to
or to

Example

If an individual saw her income increase from $50000 to $55000 per annum, and part of her response was to increase yearly purchases of amontillado from two casks to three casks, then
If infinitesimal values are considered, then a marginal value of would be, and the “marginal value” of would typically refer to

Example

Assume that, in some economy, aggregate consumption is well-approximated by
where
Then the marginal propensity to consume is