## 2 Materials and Methods
Part of the PID analyser measures the total VOC level to ppb levels with a wide dynamic range (0.1 ppb to 4000 ppm). Part of the IR contains four independent channels for selected analyses of CH\({}_{4}\), petroleum hydrocarbons (TP), CO\({}_{2}\) with a resolution of ppm. The PID analyser has two ranges: HISENS--detection limit is low ppb values; upper limit is 100 ppm (output also in \(\mathrm{\SIUnitSymbolMicro g}/\mathrm{m}^{3}\)). HISENS mode is extremely sensitive, suitable for measuring soil air contamination-near petrol pumps etc. STANDARD--detection limit 0.1 ppm upper limit 3000 ppm (output also in mg/ m\({}^{3}\)). The analyser also collects oxygen, soil temperature, temperature of the sampled gas and ambient pressure/vacuum for taking samples [17; 22]. After obtaining the measured data (CH\({}_{4}\), CO\({}_{2}\), TP, PID, O\({}_{2}\)), these were subsequently evaluated using statistical analyses:  
1. Definition of the basic statistical characteristics of each from the indicators which were observed. 2. Verification of the dependability between the examined quantities measured during identification of soil air pollution in the monitored environmental load. 3. Through regression analysis, a mathematical model has been defined for the confirmed dependencies of the studied quantities. 4. The variability of the measured variables was analysed due to changed input quantities by a non-parametric Wilcox test [23]. The data collected were analysed using descriptive statistics, calculating the basic statistical characteristics for each indicator-average, standard deviation, minimum, maximum, amount, variation coefficient [24]. Subsequently, we monitored the indicator which has the highest or lowest means and low or high variability (standard deviation). These indicators are in some cases more difficult to compare due to different units and due to different value range (min-max), that is why the coefficient of variation (CV) was defined, which expresses the deviation of measured values from the mean expressed in percentage (Table 1). High CV values as it is in the case of the indicators CH\({}_{4}\), CO\({}_{2}\), TP and PID, draws attention to high fluctuation of measured values, which can in our case be connected to the influence of categorical variables--the depth of measurement and date, which was further studied by variable analysis in other sections. The low value of CV in O\({}_{2}\) according to the temperature of the vapours suggests that we are dealing with indicators with relatively stable measured values with fluctuation up to 20% compared to the mean in Table 1.  
Using multidimensional analysis, we assessed the linear dependency of indicators in the JMP software environment [25; 26]. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Indicator** & **N** & **DF** & **Mean** & **Std Dev** & **Sum** & **Min.** & **Max.** & **CV** \\ \hline CH\({}_{4}\) mg/m\({}^{3}\) & 186 & 185 & 22.18 & 127.70 & 4125.3 & 0.01 & 1549.5 & 575.8\% \\ CO\({}_{2}\) mg/m\({}^{3}\) & 186 & 185 & 33,800.8 & 56,231.1 & 6,286,942.0 & 2339.2 & 600,011.0 & 166.4\% \\ TP mg/m\({}^{3}\) & 186 & 185 & 36.87 & 179.48 & 6857.0 & 0 & 1585.5 & 486.8\% \\ PID mg/m\({}^{3}\) & 186 & 185 & 5.80 & 18.32 & 1077.8 & 0 & 198.37 & 316.1\% \\ O\({}_{2}\)\% & 186 & 185 & 16.92 & 2.02 & 3147.4 & 8.44 & 19.14 & 11.9\% \\ Vapor temp. (\(\mathrm{\SIUnitSymbolCelsius}\)) & 186 & 185 & 25.13 & 4.41 & 4673.8 & 16.67 & 36.58 & 17.5\% \\ depth (m) & 186 & 185 & 3 & 1.00 & 558 & 2 & 4 & 33.4\% \\ \hline \hline \end{tabular} Note. N—measure number; DF—records the associated degrees of freedom (DF for short) for each source of variation; Std Dev—the standard deviation; CV—coefficient of correlation; TP—hydrocarbons overall.  
\end{table} 
Table 1: Statistical characteristics of numerical data.