A time series is a sequence of observations over regularly spaced intervals of time. For example:
- Monthly unemployment rates for the previous five years
- Daily production at a manufacturing plant for a month
- Decade-by-decade population of a state of the previous century
Components of a time series
- The long-term tendency of a series to increase or fall (upward trend or downward trend).
- The periodic fluctuation in the time series within a certain period. These fluctuations form a pattern that tends to repeat from one seasonal period to the next one.
- Long departures from the trend due to factors others than seasonality. Cycles usually occur along a large time interval, and the lengths of time between successive peaks or troughs of a cycle are not necessarily the same.
- Irregular movement
- The movement left after explaining the trend, seasonal and cyclical movements; random noise or error in a time series.