## 4 Results  
The results of the experimental verification of the model, which was based on the random forest algorithm, and taught and tested on the database presented in [16], are presented below. The verification was carried out on two independent data sets obtained as a result of testing excessively damp brick walls in two historical buildings, the description of which is provided in Section 3.  
The values of the mass moisture content \(U_{mc}\) generated by the algorithm were compared with the real \(U_{m}\) values that were obtained experimentally with the use of the gravimetric method, with the relationships between them shown in Figure 6. The obtained results indicate that the random forest algorithm correctly mapped the verification data. This is evidenced by the location of the points along the regression line, which corresponds with the ideal mapping. Moreover, satisfactory values of the linear correlation coefficient \(R\) equal to 0.801 for the Golden Gate, and 0.803 for the tenement house, were also obtained.  
Figure 5: The tenement house in Katowice: (**a**) picture of the building; (**b**) exemplary view of the basements; (**c**) a fragment of the brick wall of the basement.  
The results of the experimental verification presented in Tables 5 and 6 indicate that there is a correct identification of the validation data. This is evidenced by the fact that low median of the absolute error \(\mid\!\Delta U_{m}\!\mid\) (amounting to 1.79% for the Golden Gate in Gdansk and 1.46% for the tenement house in Katowice) and not very high (for in situ research) values of the median of the relative error \(\mid\!RE\!\mid\) (amounting to 16.70% and 13.75%, respectively) were obtained. It is also worth to note that the average values of the \(U_{mc}\) humidity content, which were identified using the RF algorithm, are close to the average \(U_{m}\) values obtained using the gravimetric method. In the case of the Golden Gate, these values are 13.82% and 14.65%, respectively, and in the case of the tenement house, the values are 9.80% and 9.83%.  
Figure 7 shows the histograms of the relative measurement error \(\mid\!RE\!\mid\) determined for the RF-generated \(U_{mc}\) values in the case of the Golden Gate (Figure 6a) and the tenement house (Figure 6b). The distribution visible in the graph (a) is rather symmetric and the relative error values \(\mid\!RE\!\mid\) are mostly in the range of 15% to 20% with the arithmetic average value of this error equal to 16.18% for the Golden Gate in Gdansk. The distribution visible in the graph (b) is right-skewed, which indicates the presence of outliers. Most \(\mid\!RE\!\mid\) values are therefore concentrated below the arithmetic mean value, and the median value of this error is equal to 13.75% for the tenement house in Katowice.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Data Set No.** & \
\begin{tabular}{c} **The Real Humidity Content** \\ \(U_{m}\) (\%)** \\ \end{tabular} &
\begin{tabular}{c} **The Humidity Content Determined** \\ **with the Use of RF \(U_{mc}\) (\%)** \\ \end{tabular} & **Absolute Error \(\mid\!\Delta U_{m}\!\mid\)(\%)** & **Relative Error \(\mid\!RE\!\mid\) (\%)** \\ \hline
1. & 13.09 & 10.45 & 2.64 & 20.17 \\ \hline
2. & 12.95 & 11.17 & 1.78 & 13.75 \\ \hline
3. & 4.97 & 5.50 & 0.53 & 10.66 \\ \hline
4. & 10.15 & 11.20 & 1.05 & 10.34 \\ \hline
5. & 10.38 & 9.96 & 0.42 & 4.05 \\