Assessment of reporting biases
Reporting biases arise when the dissemination of research findings
is influenced by the nature and direction of results (Egger 1997).
These are described in Section 10 of the Cochrane Handbook for
Systematic Reviews of Interventions (Higgins 2008). We are aware
that funnel plots may be useful in investigating reporting biases
but are of limited power to detect small-study effects. We did
not use funnel plots for outcomes where there were 10 or fewer
studies, or where all studies were of similar sizes. In other cases,
where funnel plots were possible, we sought statistical advice in
their interpretation.