Page 296 - Applied Statistics with R

296 CHAPTER 13. MODEL DIAGNOSTICS

Normal Q-Q Plot

6

4
Sample Quantiles 2 0

-4 -2

-2 -1 0 1 2

Theoretical Quantiles

The Q-Q plot looks extremely good and the Shapiro-Wilk test agrees.

shapiro.test(resid(mpg_hp_add))

##
## Shapiro-Wilk normality test
##
## data: resid(mpg_hp_add)
## W = 0.96485, p-value = 0.3706

sum(hatvalues(mpg_hp_add) > 2 * mean(hatvalues(mpg_hp_add)))

## [1] 2

We see that there are two points of large leverage.

sum(abs(rstandard(mpg_hp_add)) > 2)

## [1] 1

There is also one point with a large residual. Do these result in any points that
are considered influential?