B. Grey Literature

Beyond the traditional sources of published literature, much valuable information is available from the “grey” literature. (Some or all of this is also known as “fugitive” or “ephemeral” literature.) The concept of grey literature is evolving; some define it as any literature except peer-reviewed journals and books cited in the main bibliographic databases such as MEDLINE or Embase. A contemporary definition (not limited to HTA) is as follows:

Grey literature stands for manifold document types produced on all levels of government, academics, business and industry in print and electronic formats that are protected by intellectual property rights, of sufficient quality to be collected and preserved by library holdings or institutional repositories, but not controlled by commercial publishers, i.e., where publishing is not the primary activity of the producing body (Schöpfel 2010).

For HTA, grey literature is found in such sources as:

• Health and economic statistical reports
• Regulatory documents
• Coverage policies of health authorities and insurance plans
• Drug and biologics compendia
• Health professional association reports and guidelines
• Government monographs
• Industry (e.g., life sciences company) reports
• Market research reports for particular health care sectors/industries
• Policy and research institute studies
• Reports of special expert panels and commissions
• Conference proceedings
• Technical specifications and standards
• Working papers
• Memoranda

Much of the non-proprietary grey literature and some proprietary (for a fee) grey literature are accessible via the Internet.

The main concern with using grey literature in HTA is that it is usually not subject to peer review, and therefore may be subject to certain biases that the peer review process is designed to diminish. The benefits of including grey literature in any particular literature search are highly variable (Dundar 2006). In some instances, the available peer-reviewed sources may suffice, with little to be gained by expanding a search to include grey sources. In other instances, such as when the available peer-reviewed sources are few, outdated, or have gaps or suspected biases (e.g., publication bias), certain grey sources may strengthen the body of evidence to be considered in HTA. As such, plans for conducting literature searches should weigh the pros and cons of including grey literature, and the inclusion and exclusion criteria for searches should be designed and implemented accordingly.