[Quality assurance from the theoretical viewpoint].

The process of quality assurance is related to all measures and actions used to achieve the required quality. There is agreement about the aim and benefit of quality assurance, but the appropriate way is a matter of discussion. This paper deals from a theoretical viewpoint with basic problems of quality assurance that have not been evaluated adequately so far: a) criteria for measuring quality, b) measuring quality with adjustment for case-mix and c) evaluation of the effects of quality assurance. During the last years tools and study designs have been developed, which could contribute to the solution of these problems by a more sensible analysis, more complete and quality-controlled data and more useful evaluation studies. Further improvements in external quality assurance in surgery are to be expected, if these aspects are taken into consideration.