10.4.3.1  Recommendations on testing for funnel plot asymmetry

For all types of outcome:

 

For continuous outcomes with intervention effects measured as mean differences:

 

For dichotomous outcomes with intervention effects measured as odds ratios:

 

For dichotomous outcomes with intervention effects measured as risk ratios or risk differences, and continuous outcomes with intervention effects measured as standardized mean differences:

 

Based on a survey of meta-analyses published in the Cochrane Database of Systematic Reviews, these criteria imply that tests for funnel plot asymmetry should be used in only a minority of meta-analyses (Ioannidis 2007b).

 

Tests for which there is insufficient evidence to recommend use

The following comments apply to all intervention measures. The test proposed by Begg and Mazumdar (Begg 1994) has the same statistical problems but lower power than the test of Egger et al., and is therefore not recommended. The test proposed by Tang and Liu (Tang 2000) has not been evaluated in simulation studies, while the test proposed by Macaskill et al. (Macaskill 2001) has lower power than more recently proposed alternatives. The test proposed by Schwarzer et al. (Schwarzer 2007) avoids the mathematical association between the log odds ratio and its standard error, but has low power relative to the tests discussed above.

 

In the context of meta-analyses of intervention studies considered in this chapter, the test proposed by Deeks et al. (Deeks 2005) is likely to have lower power than more recently proposed alternatives. This test was not designed as a test for publication bias in systematic reviews of randomized trials: rather it is aimed at meta-analyses of diagnostic test accuracy studies, where very large odds ratios and very imbalanced studies cause problems for other tests.