Graphical Presentation of Trends in Rates

A variety of graphical approaches have been used to visually portray and analyze temporal trends, especially annual rates of change, in disease rates. In reviewing their own work, the authors have found that they could improve on their graphical presentations by choosing an arithmetic or logarithmic scale according to the research question being addressed, by selecting scaling ratios of the axes that allow one to detect specific rates of change, and by using uniform scaling ratios to facilitate comparisons across graphs. Different presentations of the same data can result in different impressions. The authors used rates over a 40-year period to illustrate the concepts involved and to investigate the portrayal of various rates of change. These principles extend to the plotting of odds ratios or relative risks, where the choice of an arithmetic rather than a logarithmic axis also can result in a misleading plot. The authors conclude that a graph should be designed and studied with care, clearly reflect the truth, convey information, and make a point without overemphasizing. For the particular problem addressed here, displaying temporal trends in disease rates, they see advantages in justifiable uniformity. Attention to plotting techniques in the epidemiologic and medical literature should be encouraged.