How do we make health impact assessment fit for purpose?

Progress has been made in recent years in the process of health impact assessment (HIA), including community involvement. The technical side is less well developed. A minimum requirement is that there should be some consistency or robustness, so that the outcome of an HIA does not depend just on who happens to carry it out, that it is not easily swayed by the vested interests that typically surround any project, and that it can withstand legal challenge. Validity is an important criterion, as well as repeatability, as the latter can be achieved merely by propagating errors. All types of evidence should be considered legitimate, including qualitative and quantitative methods. The quality of evidence, and its generalisability, need to be carefully assessed; we should leave behind the divisive discourse around "positivism". Typically there is less information on the links from interventions (policies or projects) to changes in determinants of health than there is on the immediate precursors of health and ill-health. A practical question is, how the best existing knowledge can be made available to HIA practitioners. Other issues are more tractable than is often thought, e.g. that an HIA has to be able to trade off positive and negative impacts to different groups of people, and that the complexity of social causation prevents clear analysis of cause and effect.