Publication and related biases.

Authors' objectives This review aims to identify systematically and appraise studies that have examined methodological issues and provide empirical evidence about publication bias and other dissemination-related biases, including biases due to the time, type and language of publication, multiple publication, selective citation of references, database index bias, and biased media attention. The review seeks to answer the following questions: What empirical evidence is available on the existence and consequences of publication and related biases? What are the causes and risk factors of publication and related biases? What methods have been developed and how useful are these methods for preventing, detecting and correcting publication and related biases?