# Data on pollution around a chemical plant, from Rao (p. 137). # Two independent samples of river water were taken, # one upstream and one downstream. # Pollution level was measured in ppm. # Entering in data: upstream <- c(24.5,29.7,20.4,28.5,25.3,21.8,20.2,21.0,21.9,22.2) downstream <- c(32.8,30.4,32.3,26.4,27.8,26.9,29.0,31.5, 31.2,26.7,25.6,25.1,32.8,34.3,35.4) boxplot(upstream, downstream) # Do the spreads seem equal across groups? # Assuming equal variances: t.test(upstream, downstream, var.equal=TRUE) # Not assuming equal variances: t.test(upstream, downstream) # A 99% CI for the difference in means: t.test(upstream, downstream, var.equal=TRUE, conf.level=0.99)$conf.int # How should we interpret this interval?