Consider an observational study investigating the relationship between infant mortality rate (Y ) versus independent variables: regime type and GDP-PPP, observed over n = 166 countries. Note that infant mortality rate is the number of infant deaths for every 1,000 live births and GDP-PPP is gross domestic product converted to international dollars using purchasing power parity rates. The levels of regime type are: authoritarian, flawed democracy, full democracy and hybrid regime. The linear regression model of interest is