What is a time series and what are the key components of a time series?

  1. What is a time series and what are the key components of a time series? How do we “de-trend” a time series data? Provide examples.
  2. What makes data stationary? How do you use Auto Correlation Function (ACF) and Partial Auto Correlation Function (PACF) to determine the “stationarity” of time series data?