## 4 Discussion
### Critical Examination of the Methodology
The process costs of the Department of Vascular Surgery and Endovascular Surgery of the University Hospital, Heidelberg were EUR 11.75/min in 2017, according to [PERSON] et al. The cost rate was achieved by means of process cost accounting, to compare a hybrid operating theatre with a conventional operating theatre. The personnel costs are given as EUR 6.74/min in the publication [14]. In a study on the efficiency of a navigation device for ENT surgery, [PERSON] et al. calculate a cost rate of EUR 16/min (including those times when the operating theatre is not being used), which is a generally recognized approach to cost calculation for modern operating theatres [17]. The abovementioned examples of cost rates indicate that the process cost rate determined in this study can be classed as realistic. Personnel costs can be seen as a good indicator. It is also noticeable that the process cost rate in this study is identical to the cost rate from the example of the University Hospital, Heidelberg. The level of the cost rate and the number of operations performed per year have an influence on the mean difference in operating time between navigated and non-navigated surgery. It defines the level at which it is worth investing in a navigation system. An increase in the cost rate or in the number of operations signifies that the minimum time by which navigated operations need to speed up may be lower. A decrease means that navigated operations require a greater time advantage over non-navigated procedures. The cost analysis applied in this study is simplified and idealized. For instance, the accounting that is conducted compares the situation of navigated operations alone with non-navigated operations alone. The cost analysis is mainly dependent on the factor of time because a longer operating time ties up more resources. The operating time in turn is heavily dependent on the surgeon's level of experience and the difficulty of the surgery. Usable mean times cannot be determined because of high dispersion, which is consistently present for all physicians' positions. The median is more stable against outliers and, in this case, is preferable to mean times. Even though experienced handling of the navigation device has a positive impact on workload by relieving stressful phases, such as when trying to locate the sphenoid bone or the base of the skull, analysis of the data shows that the navigation device is only rarely used during junior doctors' training so that routine practice cannot be developed on which specialists, for instance, might be able to build [2]. Thus, the results presented here do not coincide with those of [PERSON] et al. from 2006 and 2009. Their studies show that the incision-suture times of endoscopically functional sinus operations are shortened by up to 10 min using a navigation system. In the study from 2009, however, particular attention is paid to ensuring a comparable manifestation of the clinical picture of chronic sinusitis. This is made possible by the specific preselection of patients [2,17]. The question arises of how far a navigation system benefits an experienced surgeon. Analysis of the navigation system records shows a mean period of use of approx. 6.4 min, i.e., the system is most frequently used in the ranges of 3-6 min and 6-9 min. Although this does not entirely coincide with the prolonged periods for the operating times of consultants and the head of the department, the time is also not recovered through the use of the navigation system, which suggests that the surgeons derive other benefits from using navigation systems rather than merely the shortening of the operating time. Two further studies show that the greatest benefit of navigated operations lies in locating the frontal sinus and the spinoid sinus. A navigation system is also an immense help in verifying the cranial base, as well as in revisional surgery for chronic polypoid sinusitis, mucocele, and malignant tumors [1,18]. [PERSON] et al.