## 3 Methodology
In the next step, data for the purposes of the cost prediction model were collected. This step involved the analysis of technical and design documentation, quantity surveys, cost estimates, as well as public client queries for 135 sewerage projects. This provided data reflecting the raw values of cost predictors (as presented in Table 1), along with real-life contract net costs (excluding value-added tax) of sewerage project construction works. It is noteworthy that, through the systematic analysis of sewerage construction projects (which constituted the second phase of the research), it was observed that the previously mentioned type of runoff, whether wastewater or stormwater, did not impact the costs. From both technological and construction cost perspectives, it does not matter which type of runoff is being constructed. Therefore, this information is excluded as a predictor of costs. Due to changes in the value of money and cost variability over time, the values of contract net costs were updated for the end of the first half of the year 2023. The updated rule is provided below. \[UCC=ACC_{t}\prod_{i=t}^{n}CI_{i} \tag{1}\]  
where:  
\(UCC\)--updated contract net cost of a sewerage project;  
\(ACC_{t}\)--actual contract net cost of sewerage projects, which was awarded in the \(t\)-th half-year period between the beginning of 2018 and the end of 2022;  
\(CI_{i}\)--cost index for \(i\)-th half-year period between the beginning of 2018 and the first half of the 2023 year;  
\(n\)--stands for the first half of the 2023 year. Cost index values that were used to update contract net costs are published periodically by a Czech company, RTS(r), a developer and provider of a price information system for \begin{table}
\begin{tabular}{c c c} \hline
**Cost Predictor** & **Input Information Description** & **Value** \\ \hline Type of project & New construction or renovation & Descriptive \\ \hline Sewer pipe’s length & Size of a project, complexity of a project, quantity of works & Numerical (length) \\ \hline Type of sewer pipe material & Technical parameter, material parameter, applied solution & Descriptive \\ \hline Sewer pipe’s diameter & Technical parameter, applied solution, capacity of a sewer & Descriptive \\ \hline Average depth of trench & Technical parameter, temporary works, safety issues & Numerical (depth) \\ \hline Groundwater table level & Technical parameter, ground conditions, temporary works, safety issues & Descriptive \\ \hline Class of soil & Technical parameter, ground conditions & \\  & parameter, safety issues & Descriptive \\ \hline Number of manholes & Size of a project, complexity of a project, safety issues & Numerical (count) \\ \hline Number of crossings with other services & Complexity of a project, temporary works, safety issues & Numerical (count) \\ \hline Works on unpaved surface & Conditions of works, complexity of works, soil type waste production, quantity of works & Numerical (length) \\ \hline Works on paved surface & Conditions of works, complexity of works, multiple type waste production, quantity of works & Numerical (length) \\ \hline Debris removal distance & Conditions of works, waste management parameter & Numerical (length) \\ \hline \end{tabular}
\end{table} 
Table 1: Initial set of cost predictors. the construction industry in the Czech Republic. (It is worth mentioning that there are two main pricing systems that provide various cost information for construction cost estimation practice in the Czech Republic. These are RTS(r) and URS(r).) Based on the structural and material characteristics of sewerage systems, the cost indexes used for calculations were derived from the RTS(r) system. More specifically, price indicators that reflect the changes in costs in sewerage construction projects between 2018 and 2023 were used. The obtained cost indexes were also compared with the second pricing system, URS(r), to verify their correctness and applicability.