Time series are time-ordered observations or measurements. A time series can consist of elements equally spaced in time—for example, annual birth counts in a city during four decades—or measurements collected at irregular periods, for example, a person's weight on consecutive visits to a doctor's office. Time plots, that is, graphical representations of time series, are very useful to provide a general view that often allows us to visualize two basic elements of time series: short-term changes and long-term or secular trends.

Time series are often used in epidemiology and public health as descriptive tools. For instance, time plots of life expectancy at birth in Armenia, Georgia, and Ukraine during the years 1970 to 2003 (σee Figure 1) reveal relatively stagnant health conditions before the 1990s in ...

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