The bacterium Pneumococcus is the cause of some upper
respiratory tract infections. This can be difficult to treat, because
many strains have become resistent to antibiotics. Because
Pneumococcus can be spread by casual physical contact and
through the air, researchers are concerned that families which live in
crowded conditions may have higher levels of Pneumococcus than
families which do not. Similarly, young children may have higher
levels than parents because of the degree of contact among children in
day cares and schools. 

The researchers locate three areas of a city with many families. One
area has ``overcrowded'' conditions (housing space per family is very
small), one area has ``crowded'' conditions (housing space per family
is adequate), and one area has ``uncrowded'' conditions (housing space
per family is more than enough). In each area, 6 families are randomly
selected; each family has a mother (M), a father (F),
and three children (C1, C2, C3). A throat swab is
taken from each person in each family on a regular basis over a fixed
time period. The number of swabs which test positive for
Pneumococcus is recorded for each person; this is the outcome.

There are two factors of interest: differences in the outcome between
conditions of crowding, and differences in the outcome between persons
in each family.

These data are from Armitage and Berry, "Statistical methods in
medical research, Third edition."