Figure 1

Funnel plots demonstrating publication bias [15]. (a) Publication bias is not present, so the funnel plot should be roughly symmetrical. (b) If the plot is not symmetrical this may indicate publication bias, but there are other possible explanations. The small outlier study may be of lesser quality, which often results in exaggerated treatment effect sizes, or it may have been performed in a particularly high-risk population where the effect is large. (c) Also asymmetrical, it appears that the smaller, less precise studies are all much more positive than the larger, more precise studies. This appears a good example of publication bias. (d) If the control event rates are added to the plot, however, the interpretation may be different. Trials with the lowest control event rates demonstrate the most positive results. The intervention may work better in lower risk patients. Alternatively this could truly represent publication bias. From the funnel plot it is impossible to know. (e) The funnel plot is hollow, which is possibly publication bias of the type where significant studies are more likely to be published than those showing no difference.