[photon137]

What the author is trying to explain are the concepts of cointegration and stationarity. A useful introduction here: http://www.uta.edu/faculty/crowder/papers/drunk%20and%20dog....

[lazzlazzlazz]

Great article, thanks very much for posting it. Nobody else seems to have captured the relevant terminology: "cointegration" and "stationarity". Those perfectly captured it for me.