The file WorldBank contains data on GDP per capita (current US$), time to prepare and pay taxes (hours), percentage of urban population, gross domestic savings as a percentage of GDP, annual population growth rate, compulsory education (years), annual inflation rate, consumer prices and foreign direct investment, and net inflows as a percentage of GDP for 145 countries.

a. Using all the data as the training sample, develop a regression tree model to predict GDP per capita.

b. What conclusions can you reach about GDP per capita?