R commands and output: ## Read data and save variables. y <- matrix(scan("AUTO83B.DAT",skip=25),ncol=2,byrow=T) usmpg = y[,1] jmpg = y[,2] jmpg = jmpg[jmpg!=-999] ## Perform two-sample t-test. z = t.test(usmpg,jmpg,var.equal=TRUE) > Case 1: Equal Variances > > Two Sample t-test > > data: usmpg and jmpg > t = -12.6206, df = 326, p-value < 2.2e-16 > alternative hypothesis: true difference in means is not equal to 0 > 95 percent confidence interval: > -11.947653 -8.725216 > sample estimates: > mean of x mean of y > 20.14458 30.48101 ## Find one-tailed and two-tailed critical values. qt(.05,z$parameter) > -1.649541 qt(.025,z$parameter) > [1] -1.967268