"sleep" <-
structure(list(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")),
.Names = c("extra", "group"), row.names = c("1", "2", "3", "4",
"5", "6", "7", "8", "9", "10", "11", "12", "13", "14", "15",
"16", "17", "18", "19", "20"), class = "data.frame")