l1
| The head length of the eldest son. |
b1
| The head breadth of the eldest son. |
l2
| The head length of the second son. |
b2
| The head breadth of the second son. |
frets
data frame has 25 rows and 4 columns.
The data consist of measurements of the length and breadth of the heads of pairs of adult brothers in 25 randomly sampled families. All measurements are expressed in millimetres.
Frets, G.P. (1921) Heredity of head form in man. Genetica, 3, 193.
Whittaker, J. (1990) Graphical Models in Applied Multivariate Statistics. John Wiley.