Students of econometrics are familiar with the "spurious regression" problem that can arise with (non-stationary) time-series data.  As was pointed out by Granger and Newbold (1974), the “levels” of many economic time-series are integrated (or nearly so), and if these data are used in a regression model then a high value for the coefficient of determination (R2) is likely to arise, even when the series are actually independent of each other.  They also...