A feeding trial of rats is carried out to study weight gain as
a function of diet. 60 rats are randomized to receive one of six
diets. The six diets were defined by two factors: three ingredients
(beef, pork, and cereal) and two protein levels (low, high). Each
rat's weight was measured before and after two weeks on the diet,
and weight change was recorded.

The data set has four columns: gain, diet (numbered 1-6), ingredient,
and protein. The diet numbers correspond to 1 beef high, 2 cereal high,
3 pork high, 4 beef low, 5 cereal low, 6 pork low.

Data are from Snedecor and Cochran, Statistical Inference, 1986, p.347.