The invention is in particular directed to a data processing method for use in computer-assisted surgery, comprising the following steps: a) providing marker data which describe a spatial arrangement of at least one marker device and/or a change in a relative spatial arrangement of at least two marker devices (5, 6, 7); b) providing condition data which describe a condition for the spatial arrangement of at least one marker device and/or a condition for the change in the relative spatial arrangement of the at least two marker devices (5, 6, 7); and c) controlling a control flow of the data processing method on the basis of the marker data and the condition data.