A graphical display useful for meta-analysis

Graphical methods are frequently used in meta-analysis to summarize their results and to explore potential sources of heterogeneity across studies. In this paper, we illustrate a graphical method for meta-analysis of studies with dichotomous exposures and outcomes that complements other graphical and analytical approaches to meta-analysis. In prospective studies, the proportion of cases among the unexposed is plotted on the horizontal axis versus the proportion of cases among the exposed on the vertical axis. Contour lines for equal values of relative risk, odds ratio or risk difference and for the combined estimate of effect and its confidence interval are then superimposed on the graph. In case-control studies, the proportion of exposed controls is plotted on the horizontal axis versus the proportion of exposed cases on the vertical axis, although only the contour lines of equal odds ratios yield direct epidemiological interpretation. In these graphs, the distribution of the individual estimates of effect with respect to the contour lines offers a clue as to the adequacy of the scale of measurement used (additive or multiplicative). This graphical method also permits direct inspection of the range of disease frequency in follow-up studies and of the range of exposure in case-control studies. Its use is illustrated with the aid of 3 examples derived from the literature.