## 4 Discussion  
### Critical Examination of the Methodology  
In terms of patient selection, the criticism may be raised that a search using classification systems will yield unspecific diagnoses and forms of treatment. The ICD-10 code, as well as the OPS-301 code, will also include all subcategories. A breakdown of operations according to separate areas of the paranasal sinuses, or based on specific procedures, was rejected because this proved difficult to analyze. It was not possible to establish a definite and representative number of procedures that would still have been comparable in this respect. Furthermore, the uneven case numbers for navigated and non-navigated groups can be problematic. For instance, there are 329 navigated versus 929 non-navigated cases. This means that the mean or the median for navigated surgery is heavily dependent on the performance of the head of the department because he performs over 50% of the procedures. The other half of the operations are divided heterogeneously between surgeons with differing levels of experience. Furthermore, it was not possible to check whether the complexity of the cases is evenly distributed. It must also be assumed that the frequency of navigation usage was greater in particularly difficult procedures, which led to a poorer time result. The process cost rate for the specific process of surgery observed in this study cannot be determined using process cost accounting. This is because the expense of process cost accounting is not justified for comparing two near-identical processes that only differ in the use of an additional device. The advantage of process cost accounting over cost unit accounting is that indirect costs are identified and apportioned to so-called cost drivers [15]. These indirect costs play a secondary role in this study. According to [PERSON], process-oriented cost unit accounting is sufficient as the basis for process optimization if the transparency of indirect costs is to be ignored [10]. The cost rates generated by cost unit accounting by the business department of the Hospital University, Erlangen fully cover the process being examined in this study. Adapting the cost rates to the process time, which in this study equates to the incision-suture time, produces an appropriate, process-oriented cost rate. The cost rates for operations quoted in the literature are difficult to compare with each other because their determination is dependent on various factors and prevailing conditions [2]. [PERSON] et al. reported in 2009 that the personnel costs of an orthopedic \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & \begin{tabular}{c} **Navigated** \\ **(Min)** \\ \end{tabular} & \begin{tabular}{c} **Non-** \\ **Navigated** \\ **(Min)** \\ \end{tabular} & \begin{tabular}{c} **Difference** \\ **(Min)** \\ \end{tabular} & \begin{tabular}{c} **Cost Rate** \\ **(EUR/Min)** \\ \end{tabular} & \
\begin{tabular}{c} **Per Operation** \\ **(EUR)** \\ \end{tabular} &
\begin{tabular}{c} **Per Year** \\ **(EUR)** \\ \end{tabular} \\ \hline Case 1 & 69 & 72 & \(-\)3 & & \(-\)5.4 & \(-\)8029.0 \\ \hline Case 2 & 69 & 59 & 10 & 11.79 & 117.9 & 26,763.3 \\ \hline Case 3 & 57 & 49 & 8 & & 94.3 & 21,410.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Cost accounting. operating theatre at the University Hospital, Dresden were EUR 5.58/min. Personnel encompasses the presence of two physicians, two theatre nurses, one surgical assistant, and the anesthetist [16]. In our study, the personnel costs for the medical and \"med. tech. activity/functional activity\" for ENT surgery and anesthesia are EUR 6.24/min. This is roughly in line with the abovementioned personnel costs. [PERSON] et al. used a cost rate of EUR 10/min as their basis for calculating operating theatre costs in 2006. The Millennium Research Group Inc. from Toronto, Canada, which issued a market analysis for image-guided surgery, was quoted as a source [2].