Accountability has become a buzzword in recent years, applied in such disparate fields as education, academia, international politics and economics. For cohort studies, epidemiological literature and guidelines put forward several ideals to assure the quality of the generated data to make it accountable. Social scientific scholarship in turn has shown that in practice, such demands can lead to tensions and needs for balance 'on the ground'. We take such insights as a starting point to explore these balancing acts. Based on an ethnographic investigation into the examination and data-gathering situations and practices within a German cohort study, we suggest that 'courtesy work' is intrinsic to the development of accountable data as a foundation for the establishment and stabilization of epidemiological facts. As such, courtesy work is part of accountable care work in epidemiological studies, which is in turn a key part of the quality assurance practices that ensure the enrollment and maintenance of people in a study, as well as the production of standardized measurement results.