Residuals \(r_i\) in function of predicted values

The plot should show points scattered symmetrically around the horizontal straight line at 0, but:

• It has got a shape of a funnel, reflecting increasing variability of residuals for increasing fitted values. The variance is not constant (homoscedasticity violation).

• The smoothed line suggests that the mean of residuals becomes increasingly positive for increasing fitted values. The residuals don’t seems to have a zero-mean.