Including NRS in a Cochrane review allows, in principle, the inclusion of truly observational studies where the use of an intervention has occurred in the course of usual health care or daily life. For interventions that are not restricted to a medical setting, this may mean interventions that a study participant chooses to take, e.g. over-the-counter preparations. Including observational studies in a review also allows exposures to be studied that are not obviously ‘interventions’, e.g. nutritional choices, and other behaviours that may affect health. This introduces a ‘grey area’ between evidence about effectiveness and aetiology. It is important to distinguish carefully between different aetiological and effectiveness research questions related to a particular exposure. For example, nutritionists may be interested in the health-related effects of a diet that includes a minimum of five portions of fruit or vegetables per day (‘five-a-day’), an aetiological question. On the other hand, public health professionals may be interested in the health-related effects of interventions to promote a change in diet to include ‘five-a-day’, an effectiveness question. Because of other differences between studies relevant to these two kinds of question (e.g. duration of follow up and outcomes investigated), studies addressing the former type of question are often perceived as being ‘better’ or ‘more relevant’ without acknowledging or realizing that they are addressing different research questions. In other instances the health intervention being evaluated in the NRS will have been undertaken for a purpose other than improving health. For example, a review of circumcision for preventing transmission of HIV included NRS where circumcision had been undertaken for cultural or religious reasons (Siegfried 2003), and it was unclear whether using the intervention for health purposes would have the same effect.