The Bayley Scales of Infant Development yield scores on two indices --
the Psychomotor Development Index (PDI) and the Mental Development
Index (MDI) -- which can be used to assess a child's level of functioning
in each of these areas at approximately one year of age.  Among normal
healthy infants, both indices have a mean value of 100.  As part of a
study assessing the evelopment and neurologic status of children who have
undergone reparative heart surgery during the first three months of life,
the Bayley Scales were administered to a sample of one-year-old infants
born with congenital heart disease.  The data are contained in the data set
"heart.dat."  The variables are:

treatment:  1 = LF, 0 = other
pdi:        PDI score
mdi:        MDI score