Quantifying Non-Stationarity with Information Theory

We introduce an index based on information theory to quantify the stationarity of a stochastic process. The index compares on the one hand the information contained in the increment at the time scale tau of the process at time t with, on the other hand, the extra information in the variable at time t that is not present at time t-tau. By varying the scale tau, the index can explore a full range of scales. We thus obtain a multi-scale quantity that is not restricted to the first two moments of the density distribution, nor to the covariance, but that probes the complete dependences in the process. This index indeed provides a measure of the regularity of the process at a given scale. Not only is this index able to indicate whether a realization of the process is stationary, but its evolution across scales also indicates how rough and non-stationary it is. We show how the index behaves for various synthetic processes proposed to model fluid turbulence, as well as on experimental fluid turbulence measurements.

Automated summary

The index introduced is based on information theory to quantify the stationarity of a stochastic process by comparing information at different time scales. It provides a multi-scale quantity to explore complete dependences in the process, measures the regularity of the process, and indicates the non-stationary characteristics across scales.