## 4 Evaluation Results and Discussion
### Performance Variation across Databases  
Figure 1 shows the performance of all the models corresponding to different databases, where the red line indicates the average performance of each model. With respect to the performance variations, it is clear that, for most of the models, their performances vary significantly across databases. For _[PERSON]'s (ori)_ model, the PCC value can be as high as \begin{table}
\begin{tabular}{l c c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Model**} & \multicolumn{10}{c}{**Database**} \\ \cline{2-13}  & \multicolumn{2}{c}{W-} & \multicolumn{2}{c}{W-} & \multicolumn{2}{c}{W-SQoE-} & \multicolumn{2}{c}{} & \multicolumn{2}{c}{} & \multicolumn{2}{c}{} & \multicolumn{2}{c}{} & \multicolumn{2}{c}{} & \multicolumn{2}{c}{} & \multicolumn{2}{c}{} & \\ \multicolumn{1}{c}{_SQoE-I_} & \multicolumn{2}{c}{_SQoE-II_} & \multicolumn{2}{c}{_III_} & \multicolumn{2}{c}{_(full)_} & & & & & & & & & & \\ \hline _[PERSON]’s_[6] & Test & Test & Test & Test & Test & Test & Test & Test & Test & Test & Test & Test & Test \\ \hline _[PERSON]’s_[7] & Test & Test & Test & Test & Test & Test & Test & Test & Test & Test & Test & Test & Test & Test \\ \hline _[PERSON]’s_[11] & Test & Test & Test & Test & Test & Test & Test & Test & Test & Test & Test & Test & Test & Test \\ \hline _[PERSON]’s_[9] & Test & Test & Test & Test & **Train** & Test & Test & Test & Test & Test & Test & Test & Test & Test \\ \hline _[PERSON]’s_[8] & Test & Test & Test & Test & Test & Test & **NA** & **NA** & **NA** & **NA** & **NA** & **NA** & **NA** \\ \hline _[PERSON]’s_[12] & Test & Test & Test & Test & Test & Test & Test & Test & Test & Test & Test & Test & Test & Test \\ \hline _LSTM_[10] & Test & Test & Test & Test & Test & **Train** & Test & Test & Test & Test & Test & Test & Test & Test \\ \hline _ATLAS_[26] & Test & Test & Test & Test & Test & **NA** & **NA** & **NA** & **NA** & **NA** & **NA** & **Test & Test \\ \hline _PI203_[13] & Test & Test & Test & Test & Test & **Train** & **Train** & Test & **Train** & **Train** & Test & Test & Test & Test \\ \hline CGM [22] & Test & Test & Test & Test & **Train** & Test & Test & Test & Test & Test & **Train** & Test \\ \hline _hQPS_[27] & Test & Test & Test & Test & **Train** & Test & **Train** & Test & Test & Test & Test & **Train** & Test \\ \hline _SQI_[16] & **Train** & **Train** & Test & Test & Test & **NA** & **NA** & **NA** & **NA** & **NA** & Test & Test \\ \hline _KSQI_[28] & **Train** & **Train** & Test & Test & Test & **NA** & **NA** & **NA** & **NA** & **NA** & **NA** & Test & Test \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Evaluation setting of the databases corresponding to each model. NA: Not applicable. 0.93 in case of the _W-SQoE-I_ database but can also be as low as 0.29 in case of the _VL13_ database. More drastically, variations in PCC can be observed with _[PERSON]'s_, _[PERSON]'s_, and _biQPS_ models. On the other hand, the performances of the _LSTM_, _SQI_, and _KSQI_ models are relatively stable with PCC values in [0.63, 0.96]. From Figure 1b,c, the performances in terms of RMSE and SROCC also show the similar trend to that of PCC. In terms of the average performance, the models of _[PERSON]'s_ (both _ori_ and _mod_), _[PERSON]'s_, and _[PERSON]'s_ have the lowest average performance (i.e., PCC \(\leq\) 0.57, SROCC \(\leq\) 0.56, and RMSE \(\geq\) 0.67). Meanwhile, the highest average performance is obtained by the _LSTM_ model (i.e., PCC = 0.88, SROCC = 0.87, and RMSE = 0.38). For more details, Table 4 shows the performances of all models per database in terms of PCC. It can be seen that the highest performances are achieved by one of the eight models _LSTM_, _[PERSON]'s_, _[PERSON]'s_, _P.1203_, _SQI_, _KSQI_, _biQPS_, and _CQM_. In particular, the _LSTM_ model is very effective, offering the highest performance on six databases. In contrast, none of the _[PERSON]'s_ (both _ori_ and _mod_), _[PERSON]'s_, _[PERSON]'s_, _[PERSON]'s_, or _ATLAS_ models deliver the highest PCC value on any database.