## 4 Data Collection and Correlation Analysis
### Selection of Variables  
For SBM DEA, the selection of input and output variables is necessary before assessing DMUs. After reviewing previous research as shown in Table 2, we use two kinds of input variables: energy input and environmental input. Factors including energy consumption and share of renewable energy were used as energy inputs while variables while factors including labor productivity and capital stock were used as environmental input variables. However, as the as the data of the factor capital stock is not available, thus we use the gross capital formation productivity instead. In this research, the factor gross domestic product (GDP) is used as a desired output, while CO\({}_{2}\) emissions is used as an undesired output. Table 3 lists all the input and output variables and their units of measurement. The measurements of these variables are detailed as follows. The energy consumption is measured by ton of oil equivalent per capita and was collected from the World Bank [44]. Labor productivity is calculated as total GDP per total labor force employed, and all related figures for calculation are provided by the World Bank. The share of renewable energy in total final energy consumption is computed as total renewable consumption per total final energy consumption. Renewable consumption is collected from Eurostat, and total energy consumption is collected from the Enerdata Yearbook [45]. Gross capital formation productivity is collected from the World Bank. Data for GDP per capita and CO\({}_{2}\) emissions per capita were also collected from the World Bank. Table 4 shows the summary of collected data for the input and output variables in the time period 2013-2017. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**DMU** & **Country** & **DMU** & **Country** & **DMU** & **Country** \\ \hline
1 & Austria & 7 & Greece & 13 & Portugal \\
2 & Belgium & 8 & Hungary & 14 & Spain \\
3 & Czech Republic & 9 & Italy & 15 & Sweden \\
4 & Finland & 10 & Netherlands & 16 & Switzerland \\
5 & France & 11 & Norway & 17 & United Kingdom \\
6 & Germany & 12 & Poland & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: List of 17 selected countries (decision-making units—DMUs). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Research Title** & **Input Variable** & \multicolumn{2}{c}{**Output Variable**} \\ \hline \hline \multirow{6}{*}{[PERSON] et al. [31]} & \multirow{6}{*}{– Work populace} & – & Gross territorial local item (D) \\  & & – & Holder throughput (D) \\  & & – & Nitrogen oxide (NOx) (UD) \\  & & – & Sulfur oxide (SO\({}_{2}\)) (UD) \\  & & – & Carbon dioxide (CO\({}_{2}\)) (UD) \\ \hline \multirow{6}{*}{[PERSON] et al. [35]} & – & Capital & \\  & – & Labor & – & GHG \\  & – & Fossil fuels & – & GDP \\  & – & Renewable Energy consumption & \\ \hline \multirow{6}{*}{[PERSON] et al. [36]} & – & Labor productivity & \\  & – & Capital productivity & – & GDP \\  & – & Weight of fossil energy & – & GHG \\  & – & Share of renewable energy in GDP & \\ \hline \multirow{6}{*}{[PERSON] et al. [37]} & – & Labor productivity & \\  & – & Gross capital formation productivity & – & GDP \\  & – & Energy use & – & CO\({}_{2}\) emission \\  & – & Renewable energy consumption & \\ \hline \multirow{6}{*}{[PERSON] et al. [38]} & \multirow{6}{*}{– Metropolitan strong waste (MSW)} & – & Capital arrangement \\  & – & Metropolitan strong waste (MSW) & – & GDP \\  & – & Age & – & Populace thickness \\  & – & Age & – & Blue sulfur oxide (SOx) \\  & – & Work rate & – & Nitrogen oxide (NOx) \\  & – & & GHG \\ \hline \multirow{6}{*}{[PERSON] et al. [39]} & \multirow{6}{*}{– Capital stock} & – & GDP \\  & – & Capital stock & – & Solid waste \\  & – & Labor & – & Household refuse \\  & – & Construction land area & – & SO\({}_{2}\) emission \\  & – & Water consumption & – & Soot \\  & – & Energy consumption & – & Industrial dust \\  & – & & Waste water \\ \hline \end{tabular}
\end{table} 
Table 2: Summary of input and output variables used in previous studies.