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 problem of optimal linear filtering, smoothing and trend extraction for m-period differences of processes with a unit root is studied. Such processes arise naturally in economics and finance, in the form of rates of change (price inflation, economic growth, financial returns) and finding an...