Unit Root Processes
A unit root process is a stochastic trend in a time series where a shock to the system has a permanent effect on the level of the variable. In technical terms, it means the series is non-stationary and does not revert to a long-term mean.
If a financial asset follows a unit root process, its variance increases over time, making it impossible to predict long-term price levels accurately. This is a classic characteristic of a random walk, which is often observed in efficient, liquid markets.
For quantitative analysts, identifying a unit root is a prerequisite for proper statistical modeling, as regressing two unit root processes can lead to nonsensical, high-correlation results that do not exist. Practitioners must difference the data to achieve stationarity before applying standard econometric tools.
Ignoring unit roots leads to the overestimation of the predictive power of trading signals.