Enhancing the evidence base for health impact assessment.

Health impact assessment differs from other purposes for which evidence is collated in a number of ways, including:the focus on complex interventions or policy and their diverse effects on determinants of health;the need for evidence on the reversibility of adverse factors damaging to health;the diversity of the evidence in terms of relevant disciplines, study designs, quality criteria and sources of information;the broad range of stakeholders involved;the short timescale and limited resources generally available;the pragmatic need to inform decision makers regardless of the quality of the evidence. These have implications for commissioning and conducting reviews. Methods must be developed to: facilitate comprehensive searching across a broad range of disciplines and information sources; collate appropriate quality criteria to assess a range of study designs; synthesise different kinds of evidence; and facilitate timely stakeholder involvement. Good practice standards for reviews are needed to reduce the risk of poor quality recommendations. Advice to decision makers must make explicit limitations resulting from absent, conflicting, or poor quality evidence.