What is a time series?

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.
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