Question 1

dat <- read.csv("https://raw.githubusercontent.com/tmatis12/datafiles/main/US_Japanese_Cars.csv")
qqnorm(dat$USCars)

qqnorm(dat$JapaneseCars)

Question2

boxplot(dat$USCars, dat$JapaneseCars, names = c("US", "Japanese"), main="Boxplot of Untransformed Data")

The variance does not appear to be constant.

Question3

logUS <- log(dat$USCars)
logJP <- log(dat$JapaneseCars)
qqnorm(logUS)

qqnorm(logJP)

boxplot(logUS, logJP, names = c("US", "Japanese"), main="Boxplot of Transformed Data")

The variance appears to be the same since the interquartile sizes are similar.

Question4

Ho: μ1=μ2

Ha: μ1!=μ2

t.test(logUS, logJP, var.equal = TRUE)
## 
##  Two Sample t-test
## 
## data:  logUS and logJP
## t = -9.4828, df = 61, p-value = 1.306e-13
## alternative hypothesis: true difference in means is not equal to 0
## 95 percent confidence interval:
##  -0.6417062 -0.4182053
## sample estimates:
## mean of x mean of y 
##  2.741001  3.270957

The p value is much less than 0.05, so Ho can be rejected, which means we can conclude that Japanese cars and US cars have different fuel comsuptions.

mean(logUS)
## [1] 2.741001
mean(logJP, na.rm=TRUE)
## [1] 3.270957

The result of t-test is correct, the means of the mpg of Janpanese cars and US cars are not the same.