Testing the assumption of Normality and Homoscedasticity
A quantile-quantile graph was plotted to check the residuals follow a
normal distribution and whether the cluster of points form a straight
line was visually verified. To check that the variance of the residuals
is constant, we can plot the residuals against the fitted values and
verify visually that the cluster of points does not show any particular
trend (Picard et al ., 2012). Furthermore, the scale-location plot
and each points leverage plot were also constructed to check the
homoscedasticity assumption and the existence of possible outliers
respectively.