library(NHANES)
nh_adult <- subset(NHANES, Age >= 18)
library(stats)
library(ggplot2)
#One factor multiple levels
x<-nh_adult$Race1
y<-nh_adult$Height
lmod<-lm(y~x) 
# Normality and mean zero of residuals
qqnorm(residuals(lmod))
qqline(residuals(lmod))
plot(jitter(fitted(lmod)),residuals(lmod))
tapply(y,x,mean, na.rm = TRUE)
abline(h=0)
par(mfrow=c(3,2))
out<-tapply(y,x,qqnorm) # Each subset seems to deviate from normality at the tails.

#Equality of variances: Levene's test with deviation from median
library(car)
leveneTest(lmod,center=median)

#Pairwise testing and confidence interval
options(contrasts=c("contr.treatment", "contr.treatment"))
summary(lm(y~x))
# This is when Black (the first level) is one of the levels being compared.
#The following will give all the pairwise differences
install.packages("emmeans")
library(emmeans)
pairs(emmeans(lmod,~x))