Evidence-based public health: The importance of finding 'difficult to locate' public health and health promotion intervention studies for systematic reviews.

Systematic reviews are used to assess and summarize research, and therefore are instrumental to decision-making in public health and health promotion policy and practice. The validity of the results of a systematic review is highly dependent on the data collection methods used. This includes a systematic search to locate all relevant studies, which is often a difficult and challenging task. A great deal of public health and health promotion research has been conducted, but only some of it is available in the public domain, and overall the literature is widely dispersed. This paper aims to highlight some of the issues surrounding publication bias, including database bias, language bias and grey literature bias, and to illustrate these issues with public health examples where available. Publication bias is defined here as the publication or non-publication of research findings based on the nature and direction of results. Methodology to prevent or minimize publication bias is described, including an outline of the role of the Cochrane Health Promotion and Public Health Field (the HP&PH Field) in improving access to ‘difficult to locate’ health promotion and public health intervention studies and effectiveness reviews.