In an almost unreferenced article Carmichael (1928), writing of the period around the First World War, noted that “During the past twelve years many economic series have undergone what appears to be a permanent change in level.” These are prescient words that are widely applicable today....

The development of the methods of correlation and regression analysis at the turn of the 20th century, led to their use in attempting to identify relationships between economic variables. However, caution was soon expressed that correlating series with ‘secular’ trends was likely to be...