These data were collected as part of the Six Cities study, which 
studied the effects of air pollution in adult and child populations 
in six selected U.S. cities. These data are for 32 children only, 
and were collected to examine the association between maternal
smoking and child respiratory health.  Each child was examined
once per year at a clinic at ages 9, 10, 11, and 12 for evidence
of wheezing (1 = present, 0 = absent).  There are thus up to four 
follow-up measures for each participant, although there are missing 
data.  In addition the mother's smoking status was recorded (0 = none,
1 = moderate, 2 = heavy).

For more on these data, see Lipsitz, Laird, and Harrington (1992, 
Applied Statistics, 41:203-213) and the references therein.