>‘The simplest and most commonly used method to detect publication selection is an informal examination of a funnel plot’ (Sutton et al. 2000a: 1574). A funnel

>‘The simplest and most commonly used method to detect publication selection is an informal examination of a funnel plot’ (Sutton et al. 2000a: 1574). A funnel graph is a scatter diagram of precision versus estimated effect (such as estimated elasticities, regression coefficients or partial correlation coefficients). Precision is best measured by the inverse of the standard error (1/Se). > As the name suggests, the expected shape is an inverted funnel — in the absence of publication selection. When there is no publication selection, estimates should vary randomly and symmetrically around the ‘true’ population effect. Because small-sample studies with typically less precision form the base of the graph, the plot will be more spread out there than at its top. However, it is the graph’s symmetry (or its absence) that is crucial for assessing publication selection (see Figure 1).