A dairy scientist recently conducted an on-farm trial to examine
the effect of grain supplement on milk yield.  She went to two {\tt
farm}s selected at random from Columbia County and located a number of
cows to include in the study.

Cows had to be in ``lactation'', that
is, have recently had a baby calf and be actively producing milk.
Every effort was made to have the same number of animals having their
first calf ({\tt lact=1}) vs. later calves ({\tt lact=2}).  [It is
known that animals in their first lactation can behave rather
differently than more mature animals.]  In addition, {\tt past} milk
production was scored as {\tt lo} or {\tt hi} based on records kept by
the farmers.

Two treatments ({\tt trt}) were applied, the {\tt old} feed and a {\tt
new} feed with added grain supplement.  Milk production was measured
after a week for the subsequent 2 weeks, and converted to units of
thousands of pounds per year ({\tt yield}) for comparison purposes.

Your task is to determine the effect of added grain supplement on milk
yield, in light of other information about the animals.
