The series value X is plotted on the vertical axis and time t on the horizontal axis. Continuous, where we have an observation at every instant of time e.g. lie detectors, electrocardiograms. We denote this as Xt. We want to increase our understanding of a time series by picking out its main features. One of these main features is the trend component.
Trend is a long term movement in a time series.
If these averages change with time we can say that there is evidence of a trend in the series. There are also more formal tests to enable detection of trend in time series. It can be helpful to model trend using straight lines, polynomials etc. See also time series . See also cyclical component .