expectancy

Expectancy is a broad term referring to what is anticipated to occur in the future, or to the duration one expects to cover. The word is used across disciplines, often with technical meanings tied to probability, statistics, demography, psychology, and medicine. In general, it expresses an expectation about outcomes, timing, or length of existence.

In probability and statistics, expectancy is commonly used to mean the expected value or mathematical expectation. It is the probability-weighted average of all possible values of a random variable. For a discrete variable, it is the sum of each value multiplied by its probability; for a continuous variable, it is the integral of the variable with respect to its probability density. The concept underpins decision making, risk assessment, and financial theory.

In demography and epidemiology, life expectancy refers to the average number of years a person is expected to live under prevailing mortality rates. It can be reported at birth, at a given age, or as health-adjusted life expectancy. Life expectancy is a population metric rather than a forecast for individuals, and it varies with geography, time, and public health conditions.

In psychology and organizational behavior, expectancy theory describes motivation as a function of expectancy (the belief that effort will lead to performance), instrumentality (that performance will lead to outcomes), and valence (the value of those outcomes).

In clinical contexts, expectancy may describe prognosis or anticipated survival, often expressed as median survival or similar estimates, and it can be influenced by disease severity, treatment options, and patient condition. See also expectation.