Learning Statistics with R - A tutorial for psychology students and other beginners, 2018a

Standardized residuals
0.0 0.5 1.0 1.5
78
Scale−Location
81 36
50 60 70 80
Fitted values
lm(dan.grump ~ dan.sleep + baby.sleep)
Figure 15.15: Plot of the fitted values (model predictions) against the square root of the abs standardised
residuals. This plot is used to diagnose violations of homogeneity of variance. If the variance is really
constant, then the line through the middle should be horizontal and flat. This is one of the standard
regression plots produced by the plot() function when the input is a linear regression object. It is
obtained by setting which=3.
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that only make sense if you understand the maths at a low level 14 You don’t need to understand what
this means (not for an introductory class), but it might help to note that there’s a hccm() function in
the car() package that does it. Better yet, you don’t even need to use it. You can use the coeftest()
function in the lmtest package, but you need the car package loaded:
> coeftest( regression.2, vcov= hccm )
t test of coefficients:
Estimate Std. Error t value Pr(>|t|)
(Intercept) 125.965566 3.247285 38.7910