The coefficient of coherence is defined for bivariate weakly stationary stochastic processes which have spectral distributions dominated by a fixed Lebesgue-Stieltjes measure. This quantity is shown to possess two of the important properties which make the ordinary correlation coefficient a desirable measure of linear regression for pairs of random variables. This provides a justification for the already common use of the coefficient of coherence as a measure of linear-regression for pairs of stationarily correlated, weakly stationary time series.
"On the Coefficient of Coherence for Weakly Stationary Stochastic Processes." Ann. Math. Statist. 35 (2) 532 - 549, June, 1964. https://doi.org/10.1214/aoms/1177703553