Investigating and Dealing with Publication Bias and Other Reporting Biases

A p value, or the magnitude or direction of results, can influence decisions about whether, when, and how research findings are disseminated. Regardless of whether an entire study or a particular study result is unavailable because investigators considered the results to be unfavorable, reporting bias in a meta-analysis may occur when available results differ systematically from missing results. This reinforces the need for review authors to search or consult multiple sources that include bibliographic databases, trials registers, manufacturers, regulators, and study authors or sponsors where or through whom study reports and results may be located. Unless prospective approaches to meta-analysis can eliminate the potential for bias due to missing results, review authors should formally assess the risk of bias in their review. Several approaches can facilitate such assessment: tools to record selective reporting of results, ascertaining qualitative signals that suggest not all studies were identified, and the use of funnel plots to identify small-study effects, one cause of which is reporting bias.