Variation matters and should be included in health care research for comparison of outcomes.

BACKGROUND: Health care is provided under the conditions in which people live and under the rules and regulations of a prevailing health system. As a consequence, 'local' circumstances are an important determinant of the actual care that can be provided and its effects on the health of individuals and populations. This plays in particular, but not exclusively, a role in community-based primary health care. Although this is generally accepted, there is little insight in the impact of the setting and context in which health care is provided on the outcome of care. Aim This paper argues the case to use this natural variation within and between countries as an opportunity to be used as a form of natural experiment in health research. Arguments We argue that analysing and comparing outcomes across settings, that is comparative outcomes of interventions that have been performed under different health care conditions will improve the understanding of how the real-life setting in which health care is provided - including the health system, the socio-economic circumstances and prevailing cultural values - do determine outcome of care. Recommendations To facilitate comparison of research findings across health systems and different socio-economic and cultural contexts, we recommend a more detailed reporting of the conditions and circumstances under which health research has been performed. A set of core variables is proposed for studies in primary health care.