Time series analysis is very common; you run across it every day from stock market price trends to GDP graphs. It is intuitive and so pervasive that it feels awkward even giving it a name. Time series is simply ordering data values chronologically and analyzing the results for patterns. The order should be at equally-spaced intervals even when there is no data for the interval.

Patterns can be discovered by the time of year (seasonal), by the business cycle, from values increasing or decreasing over time (trends), and even by the time of the day. Identified patterns can then be used in forecasting future values over time. We will look at the time series forecasting technique later in the book.