Referencing for reviews: How to distinguish unwillingness from mission impossible.

Selecting the most relevant literature for a state-of-the-art review article may result in the unintentional exclusion of valuable contributions, despite the most genuine intention of comprehensiveness. Some of these omissions, however, can be sensibly justified by a number of factors, such as language barriers, lack of abstract and/or online full-text article, type of manuscript and date of publication, whose concurrence makes simply unfeasible the gathering of the most basic information, indispensable for the citation. In this case, even the oceanic collection of scientific contributions contained in the database Medline may become underpowered.