aikamuuttuva

An aikamuuttuva (Finnish for "time variable") is a variable whose value changes as time passes. In mathematical notation it is often denoted as y(t) for continuous time or y_t for discrete time, reflecting time as the independent variable in a model of a dynamic system.

Time dependence is central to many scientific and engineering frameworks. In differential equations and dynamical systems, quantities are expressed as functions of time to describe how systems evolve. In statistics and econometrics, time-varying covariates capture effects that evolve during the observation window, such as temperature, revenue, or health status. In survival analysis, covariates may change between events and are treated as time-dependent.

In time series analysis, an aikamuuttuva is the observable indexed by time, forming sequences of measurements such as stock prices, sensor readings, or ecological counts. In physics and engineering, signals are treated as time-dependent quantities in the time domain. In computer simulation and control theory, time steps advance the state of the system and drive the evolution of simulations.

Practical challenges include irregular sampling, missing time stamps, and alignment across data sources. Analysts often use interpolation, resampling, or state-space models to handle time-varying data. Selecting appropriate temporal granularity and addressing non-stationarity are important considerations when modelling aikamuuttuvat.

See also time series, time-dependent models, time-varying covariates, and dynamic systems.