How low can you go? Toward a Hierarchy of Grey Literature

In recent times health sciences librarians have become increasingly involved in working closely with researchers who are undertaking literature reviews (often, but not always, systematic literature reviews). Due to the evidence-based push throughout the medical and health practice area, strong database searching skills have become essential to locate and identify the research necessary to inform practice. These are skills that librarians already have. In hospitals and universities, librarians have been heartened to find their expertise openly acknowledged as their role dovetails neatly into a position of strength. Working with researchers we find our opinions are sought and valued, and this extends to questioning our knowledge of the grey literature. While searching the published (black) literature is a given, there is a growing recognition that grey literature should be included to fully reflect the existing evidential base. Herein lies the rub. Without a clearly established hierarchy, what grey literature is admissible: particularly if we consider grey resources other than the unpublished equivalents of traditional studies and trials? Literature reviews need to include the most significant research available. And that might mean conference papers, reports, legislation or working papers - all grey. This paper examines the variety, relevance, advantages and disadvantages of grey literature and where it might ‘sit’ in the overall research picture.