Systematic review of publication bias in studies on publication bias

Publication bias is a well known phenomenon in clinical literature,1 2 in which positive results have a better chance of being published, are published earlier, and are published in journals with higher impact factors. Conclusions exclusively based on published studies, therefore, can be misleading.3 Selective underreporting of research might be more widespread and more likely to have adverse consequences for patients than publication of deliberately falsified data.1 We investigated whether there is preferential publication of positive papers on publication bias. We identified studies that assessed the impact of publication bias in Medline (January 1993 to October 2003) using the search terms “publication bias”, “citation bias”, “language bias”, location bias”, “reference bias”, or “multiple publication bias”. We also searched the references of …