Publication bias is a systematic underestimate or an overestimate of the underlying beneficial or harmful effect due to the selective publication of studies. Publication bias arises when investigators fail to report studies they have undertaken (typically those that show no effect). Authors should suspect publication bias when available evidence comes from a number of small studies, most of which have been commercially funded.
A number of approaches based on the examination of the pattern of data are available to help assess publication bias. The most popular of these is the funnel plot. All, however, have substantial limitations and authors of reviews and guideline panels must often guess about the likelihood of publication bias. Again, there is no substitute for judgment.
Note that selective reporting of outcomes should be dealt with in the assessment of the individual studies.
Guideline panels or authors of systematic reviews should consider the extent to which they are uncertain about the magnitude of the effect due to selective publication of studies and they may downgrade the quality of evidence by one or even two levels. As there are no validated decision rules to do so it is important to provide a narrative justification of the final decision on this issue after consultation of the experts.
Trials registries are in principle compulsory now and can play a major role in detecting selective reporting.