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.