Stationarity

Understanding stationarity

Stationarity is a property of time series data that indicates the absence of change in the mean and in the variance over time. A stationary time series will be horizontal, meaning it has no trend, and it will change always within a certain range.

Stationarity is an assumption used by some forecasting methods, so it's important to know how to identify it before applying those methods.

Identifying stationarity

Although there are statistical methods to identify stationarity, it is usually easy to visually inspect time series data to see if it's stationary or not.

Let's first try the visual method to have a better intuition of the concept of stationarity.

The line chart above represents the number of airline passengers over time. As we can see, there is an upwards trend since ...