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strategy projects multi-component programmes.

For the purposes of this paper, an intervention is defined as a set of actions with a coherent objective to bring about change or produce identifiable outcomes. These actions may include policy, regulatory initiatives, single strategy projects or multi-component programmes. Public health interventions are intended to promote or protect health or prevent ill health in communities or populations. They are distinguished from clinical interventions, which are intended to prevent or treat illness in individuals. Context refers to the social, political and/or organisational setting in which an intervention was evaluated, or in which it is to be implemented. The contextual characteristics that are relevant vary with the type of intervention. Important contextual characteristics for a public health intervention might include factors in the political and organisational environment and socioeconomic or demographic features of the population.

Evaluation is a process of determining the value or worth of something by judging it against explicit, predetermined standards.7Evidence comprises the interpretation of empirical data derived from formal research or systematic investigations, using any type of science or social science methods. This definition of evidence is purposefully circumscribed to articulate the scope of this paper. In our consideration of evidence-based practice, we focus on evidence about likely consequences of interventions, such as effectiveness and cost effectiveness, not evidence about need for services. Thus, we distinguish between data on the cause or scale of a health problem (aetiological studies and needs assessment) and evidence on the implementation and outcomes of interventions. This paper deals with the latter.

Until recently public health epidemiology was chiefly concerned with aetiological hypotheses, rather than evaluative hypotheses. Intervention evaluation has its origins in the social sciences, notably education and psychology.8,9 To strengthen the criteria for appraising evaluative research in public health we have drawn upon a broad-based literature beyond the fields of epidemiology and evidence-based medicine. We acknowledge, however, the limitation of relying on the English language literature.