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Table 10.4.a: Possible sources of asymmetry in funnel plots

Adapted from Egger et al. (Egger 1997a)).

1. Selection biases: 

  •  Publication bias:

  • Delayed publication (also known as ‘time-lag’ or ‘pipeline’) bias.

  • Location biases:

    • Language bias;

    • Citation bias;

    • Multiple publication bias.

  •  Selective outcome reporting.

2. Poor methodological quality leading to spuriously inflated effects in smaller studies:

  •  Poor methodological design;

  • Inadequate analysis;

  • Fraud.

3. True heterogeneity:

  • Size of effect differs according to study size (for example, due to differences in the intensity of interventions or differences in underlying risk between studies of different sizes).

4. Artefactual:

  •  In some circumstances (see Section 10.4.3), sampling variation can lead to an association between the intervention effect and its standard error.

5. Chance.