Data consist of random subsample of 1752 females from the Collaborative
Perinatal Project.  Thanks to Mike Pennell for providing the data.


The variable descriptions are as follows:

personid = index of girl number (repeated measurements on each girl)

wt = weight in grams

age = age in years

smoke = 1 if never smoker, 2 if quit smoking prior to registration, 3 if
current smoker

over_wt=1 if weight >= 95th percentile, 2 if less than 95th percentile

Note that over_wt is missing (equal to -999 is the flag) at 0 and 1 
years, because there are no standards at those young ages