All annual trade figures, especially those of manufacturing countries, are subject to wide fluctuations, owing to recurring periods of trade depression of longer or shorter duration.

Consequently, statements of trade progress or decline, based on the comparison of the totals for single years, however carefully chosen, are most untrustworthy, and general conclusions based on such statements are wholly fallacious. A true estimate of relative trade progress or decline can only be obtained by plotting the figures for a series of years in curve diagrams, or by comparing figures for 5 or 10 year trade averages.