Five years has been the average, and indeed the typical, duration of a full cycle in the British economy since the war. For that reason it has been taken as a convenient time span when considering medium-term developments such as the trend rate of growth of output or productive potential. If we now look back five years from 1987 the annual growth rate averages about 3½ per cent, suggesting a marked improvement in trend since the 1970s. This may be the correct conclusion to draw, but it cannot be accepted uncritically since the pattern of five-year cycles seems to have changed.