While the “scientific community” holds to the principle that all public health must be evidence based, in practice the effectiveness of many health promotion interventions and programmes is not properly assessed.¹ Among the reasons for this situation, the lack of an adequate control group is commonly an important obstacle for a careful evaluation of effects. An equivalent control group is rarely available to assess the impact of large scale health promotion campaigns or new policies, which are often dependent on time trends comparisons. While time trends sometimes show impressive changes, often historic trends or other factors influencing outcome are not properly taken into account, resulting in a very simplistic appraisal of the programme’s utility. As a consequence, the interpretation of changes seen on timelines becomes a rather subjective appreciation.

Another main barrier to impact evaluation is the complexity of many health promotion interventions, especially those targeting human behaviour change. Unlike therapeutic and diagnostic procedures, preventive interventions that aim to promote healthy lifestyles focus on knowledge, attitudes, and beliefs, as the “predisposing factors” of a certain behaviour.² These factors are deeply rooted in the social and cultural context; …