11.3 Residual plots

• Look for patterns / non-randomness in residual plots.

Example:

data_2 %>%
  mutate(
    pred = predict(fit_2),
    resid = residuals(fit_2)
  ) %>% 
  ggplot(aes(pred, resid, color = factor(z))) +
  geom_hline(yintercept = 0, color = "white", size = 2) +
  geom_point() +
  theme(legend.position = "bottom") +
  labs(
    title = "Residual vs. predicted value",
    x = "Predicted value",
    y = "Residual",
    color = "Treatment"
  )

## Warning: Using `size` aesthetic for lines was deprecated in ggplot2 3.4.0.
## ℹ Please use `linewidth` instead.
## This warning is displayed once every 8 hours.
## Call `lifecycle::last_lifecycle_warnings()` to see where this warning was
## generated.