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Figure 2 | Critical Care

Figure 2

From: Bench-to-bedside review: Avoiding pitfalls in critical care meta-analysis – funnel plots, risk estimates, types of heterogeneity, baseline risk and the ecologic fallacy

Figure 2

Demonstration of Simpson's paradox occurring if results from two clinical trials are simply pooled rather than subjected to meta-analysis. The slope of each vector represents a mortality rate (deaths/total number of patients) in patient groups taking Drug 1 (dark lines) and Drug 2 (light lines). OA and OC represent the results of one trial, and AB and CD the other trial. OA has the lesser slope, meaning Drug 1 is superior in this trial. Similarly, AB and CD demonstrate Drug 1 is superior. If the data are simply pooled, the overall effect slope (of lines OB and OD) is paradoxically reversed. The same diagram can be used to demonstrate Simpson's paradox due to subgroup effects in a single nonrandomised trial, such as that in Table 2. Here OA and OC represent results from one stratum of the trial, and AB and CD the other. When the confounding effect of stratum is ignored, Drug 2 is paradoxically superior. Adapted from [39].

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Critical Care

ISSN: 1364-8535