Health impact assessment differs from other purposes for which evidence is collated in a number of ways, including:
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.