a numeric vector giving the protein content of the milk.
Time
a numeric vector giving the time since calving (weeks).
Cow
an ordered factor giving a unique identifier for each cow.
Diet
a factor with levels
barley,
barley+lupins, and
lupins identifying the diet for each cow.
Details
Diggle, Liang, and Zeger (1994) describe data on the protein content
of cows' milk in the weeks following calving. The cattle are grouped
according to whether they are fed a diet with barley alone, with
barley and lupins, or with lupins alone.