*4.1. Data Description*

The data source of our study is mainly from the World Bank, and we particularly select variables including GDP per capita (constant 2010 US \$) of different countries and regions, their total population, total capital (constant 2010 US \$), labor population (total number), total enrollment rate of higher education, GDP generated by per unit energy consumption (constant 2011 PPP US \$ per kg oil equivalent), and ratio of renewable energy.

We mainly choose relevant data during the period 1990–2015. This is driven by two reasons, the first is the United Nations Development Program had not started to collect data about the average years of schooling worldwide before the year 1990, and the second is that the statistics after the year 2015 have not been updated as of April 2019. Furthermore, we kept 187 countries and regions, which are among the 189 member states of the World Bank and also among the 193 member states in the United Nations. We excluded from our sample countries that deviate from general patterns of economic development, such as Kashmir and some post-war Middle Eastern countries. By doing this, we ensured our sample was representative, thereby improving the quality of our dataset in order to get more accurate results.
