Typically, the only new data generated in health impact assessments is through qualitative data collection. This may include focus groups, key informant interviews, participatory observation, etc. It is often not practical or necessary to generate new quantitative data. Existing data from the literature is often sufficient. Systematic reviews of available research are an effective way to gather information. For example, the U.S. Task Force on Community Preventive Services (http://www.thecommunityguide.org) offers evidence-based recommendations for programs and policies to promote population health. A list of other key evidence-based review sites are listed below.
Systematic reviews are useful for HIA but their utility is limited by fundamental differences between the focus of systematic reviews and that of HIA. Systematic reviews typically assess the efficacy of planned interventions. Unlike HIA they do not typically examine unintended consequences and downstream effects. Thus, their applicability for understanding health effects across sectors is often limited.
Systematic reviews also have different standards of evidence than HIA. The threshold of evidence for ascertaining the efficacy of planned interventions is necessarily higher than the threshold for evidence in HIA, where effects are often indirect and uncertain. A finding of “no significance” in a systematic review results in no action, whereas a finding of “no significance” in HIA may result of a potentially harmful policy or project being approved. One cross-sectional study may be all an HIA has to guide a particular analysis and recommendation, but systematic reviews usually require consistent results from multiple, well controlled research trials.