## 2 Materials and Methods  
The retrospective study included patients who underwent surgical treatment for chronic sinusitis between 1 January 2012 and 31 December 2016. A total of 1158 cases with complete data records were identified for this period. Navigation was used in nearly one-third of these cases. A group of patients with navigated sinus surgery and a second group without navigation were formed. Table 1 shows the age and gender distribution in the two groups. Table 2 lists the number of cases by year, divided into the defined groups. This gives a total mean of 227 cases per year. The number of navigated operations increased steadily during this period. The 51- to 60-year-old patients form the largest group; the patients' mean age is 48.97 years. Figure 2 provides a clear picture of the age structure. In terms of methodology, the patients were recorded using the patient data administration system, Meierhofer Clinical Competence (MCC, Meierhofer AG, Munich, Germany), from Meierhofer AG. The cases are recorded according to their ICD-10 diagnosis J32 and classified based on OPS procedures 5-22 (1-9). The navigated cases are additionally identified by OPS Code 5-988. Patients in whom a malignant tumor of the nasal or paranasal sinus or of the middle ear was detected by histology, as well as patients who simultaneously underwent septoplasty, were excluded. The incision-suture time defines the time between the first and the last step of an operation performed by the surgeon. In terms of content, the incision-suture times of both groups and cost analysis are compared. An appropriate cost estimate for sinus surgery is defined for this purpose. The operations were performed by several surgeons with different levels of experience. Table 3 lists the distribution of physicians according to seniority. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Period**} & \multicolumn{4}{c}{**01.012.2012–31.12.2016**} \\ \cline{2-5}  & **Total** & **Group without Navigation** & **Group with Navigation** \\ \hline
**Number of cases (_n_)** & 1158 & 829 & 329 \\
**female (_n_)** & 448 & 339 & 109 \\
**female (\%)** & 38.69\% & 40.89\% & 33.13\% \\
**male (_n_)** & 710 & 490 & 220 \\
**male (\%)** & 61.31\% & 59.11\% & 66.87\% \\
**Mean age (a)** & 48.97 & 48.55 & 50.04 \\
**Age: min/max** & 8/89 & 8/87 & 8/89 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Characterization of the total sample. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & **2012** & **2013** & **2014** & **2015** & **2016** & **Total** & **Mean (_n_/Year)** \\ \hline
**With navigation** & 15 & 38 & 76 & 111 & 89 & 329 & \\
**Without navigation** & 217 & 174 & 157 & 137 & 144 & 829 & \\
**Total** & 232 & 212 & 233 & 248 & 208 & 1158 & 227 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Case numbers by years. Patients with chronic rhinosinusitis (with and without polyps) underwent surgery. The diagnosis was made via nasal endoscopy, by means of imaging techniques such as CT scans. A previously unsuccessful attempt at conservative treatment was also required. As regards content, the study deals with the subprocess of surgery to treat chronic rhinosinusitis, in particular the incision-suture time of navigated vs. non-navigated surgery, as documented in the MCC program. This measure of time defines the process being studied and is also the process parameter that has a major influence. The second important process parameter is the activity quantity, i.e., the number of operations performed per year. In the MCC program, there are no noticeable differences in the preparation time between navigated and non-navigated operations because the navigation set-up is simple, is integrated into surgical routine practice, and can be managed within the time taken for regular surgical preparations. Means and medians are formed from the incision-suture times for the analysis, and for calculating process costs.