The data in "heartbeats.txt" is from a paper
of Lee Salk (The role of the heartbeat in the relation between
mother and infant, Scientific American, 1973, 228(5), 24--29).
Randomly chosen newborns heard continuously
the recording of heartbeats of
a grown-up in the first days of their life.
The gain in weight between the second and fifth day has
been recorded. The variables are treatment ("yes" refers to babies
which heard heartbeats, "no" refers to the control group),
wghtincr (increase in weight) and wghtcls ("1" refers to a birth weight
below 3000g, "2" refers to a birth weight between 3000g and 3500g,
"3" refers to a birth weight above 3500g).