This dataset comes from the "Jaws" example in the second volume of worked
examples that is included with OpenBUGS.  The dataset originally was cited in
Elston and Grizzle (1962).  It contains repeated measurements of ramus 
(jaw) bone height on a cohort of 20 boys over an 18 month period.  Each boy
was measured at ages 8.0, 8.5, 9.0, and 9.5 years old.  Each row of the
dataset contains the four measurements on one boy.