> dt
   X1 X2 X3
1   1  1 58
2   1  1 45
3   1  1 47
4   1  2 61
5   1  2 65
6   2  1 31
7   2  1 35
8   2  1 29
9   2  2 43
10  2  2 51
11  2  2 49
12  3  1 45
13  3  1 55
14  3  2 31
15  3  2 37
16  3  2 37
17  4  1 78
18  4  1 83
19  4  2 36
20  4  2 34
21  4  2 34
> dput(dt)
structure(list(X1 = structure(c(1L, 1L, 1L, 1L, 1L, 2L, 2L, 2L,
2L, 2L, 2L, 3L, 3L, 3L, 3L, 3L, 4L, 4L, 4L, 4L, 4L), .Label = c("1",
"2", "3", "4"), class = "factor"), X2 = structure(c(1L, 1L, 1L,
2L, 2L, 1L, 1L, 1L, 2L, 2L, 2L, 1L, 1L, 2L, 2L, 2L, 1L, 1L, 2L,
2L, 2L), .Label = c("1", "2"), class = "factor"), X3 = structure(c(58,
45, 47, 61, 65, 31, 35, 29, 43, 51, 49, 45, 55, 31, 37, 37, 78,
83, 36, 34, 34), .Dim = c(21L, 1L))), .Names = c("X1", "X2",
"X3"), row.names = c(NA, -21L), class = "data.frame")