Chapter 4. Understanding Time Series
A time series is a form of data that has a temporal dimension and is easily the most iconic form of financial data out there. While a single stock quote is not a time series, take the quotes you get every day and line them up, and you get a much more interesting time series. Virtually all media materials related to finance sooner or later show a stock price gap; not a list of prices at a given moment, but a development of prices over time.
You'll often hear financial commenters discussing the movement of prices: "Apple Inc. is up 5%." But what does that mean? You'll hear absolute values a lot less, such as, "A share of Apple Inc. is $137.74." Again, what does that mean? This occurs because market participants are interested in how things will develop in the future and they try to extrapolate these forecasts from how things developed in the past:
Most forecasting that is done involves looking at past developments...