Accurately measuring physical activity in epidemiological research is, of course, central to the internal validity of the research. Ultimately, getting this component of the work right is a prerequisite for successfully discerning the relation between activity and health outcomes, for setting efficacious and reasonable guidelines, for discerning causal factors for activity choices, and for intervening to improve activity levels within our communities. However, measuring a behaviour as complex and multifaceted as activity is, at best, challenging. Physical activity epidemiology relies heavily on questionnaire methods, generally to characterise easy to recall structured movement during exercise, sport, and work. Self reporting activity through questionnaires is cognitively difficult for adults and much more so for children, the elderly, and other subgroups. Questionnaires are also prone to various degrees of measurement error depending on the facet of activity queried and the time period considered. Assumed error in self reports is (somewhat) countered with large sample sizes so, despite shortcomings and given the lack of feasible alternatives, the reliance on questionnaires remains the most common assessment technique in epidemiology.¹

Recently, the methodological gap between accuracy and …