[Interpretation of research on patient volumes].

Healthcare research into patient volumes and volume-outcome relations can have far-reaching consequences. By means of a thought experiment, five core elements of valid research into patient volumes are identified: variation in numbers of patients, scalability of capacity, comparability regarding quality of care, comparability regarding groups of patients, and statistical precision. Practical challenges are described for each of these aspects as well as their possible impact on research quality. This is illustrated using the example of volume-outcome relations in the field of pediatric cardiac surgery.