sleep <- data.frame(extra = c(0.7, -1.6, -0.2, -1.2, -0.1, 3.4, 3.7,
0.8, 0, 2, 1.9, 0.8, 1.1, 0.1, -0.1, 4.4, 5.5, 1.6, 4.6, 3.4),
group = structure(c(1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 2, 2,
2, 2, 2, 2, 2, 2, 2), .Label = c("1", "2"), class = "factor"))