Subsequently, Cochrane and Orcutt (1949) made the important
point that the major consideration in the analysis of stationary time series was the
autocorrelation of the error term in the regression equation and not the autocorrelation
of the economic time series themselves. In this way they shifted the focus of attention to
the autocorrelation of disturbances as the main source of concern. Although, as it turns
out, this is a valid conclusion in the case of regression equations with strictly exogenous
regressors; in more realistic set ups where the regressors are weakly exogenous the serial
correlation of the regressors are also likely to be of concern in practice. See, for example,
Stambaugh (1999).