While resources such as http://www.health-evidence. ca, provide synthesized, high quality research evidence relevant to public health practice, coverage of health topics is not equal [25] . Compared to clinical review literature, there is far less population health systematic review literature [6] and within public health, a number of topic areas are without a solid base of review evidence evaluating the effectiveness of interventions (e.g., environmental health, social determinants of health) [18, 26, 27] . Public health studies are difficult to design and results are drawn from natural experiments (e.g. one health unit adopting a program compared to another health unit), thus fewer studies have been developed on the effectiveness of public health interventions compared to randomized controlled trials on medical treatments [28, 29] . However this lack of review-level evidence doesn't necessarily indicate a lack of evidence [30] . There may be primary research or other forms of evidence that can inform decisions but which may not yet have been synthesized. In cases where there are no eligible studies available to be reviewed for a particular topic, the result can be an "empty review", meaning that no studies were located which met the inclusion criteria to answer the question for review. Empty reviews can go unreported but may in fact be useful since empty reviews indicate interest in an area, highlight gaps, and offer a snapshot of the state of research evidence at the time of publication [31] . Even in light of a lack of evidence, or poor reporting of the evidence, decision makers can and should take an informed approach to having insufficient evidence [30] . An informed approach may be needed in topic areas that demonstrated a lack of review-level evidence, such as dental health, environmental health, food safety and inspection, and seniors' health, and particularly for public health priority areas such as healthy communities and social determinants of health. Consequently, the best literature may be sparse or of low quality in these particular topic areas. In these areas, new systematic review literature is needed to inform practice and policy decision making. It is unclear at this time whether gaps pertain only to reviews lacking in these topic areas, or whether there is a corresponding lack of primary studies as well, hindering the production of reviews. In some instances it may be necessary to first develop the primary study base in order for studies to be available for synthesis in a systematic review. Future reviews should be conducted on these broad topic areas for which review-level evidence on the effectiveness of interventions is lacking. Funding organizations should generate calls for syntheses to address these gap areas, while non-government and public health organizations should provide feedback on the lack of evidence in areas of interest to them to potential funders. Funding priorities for syntheses should reflect those areas which are priorities for public health decision makers both in Canada and internationally.