The file "Kids_n_crowns_data" has one row for each crown lifetime;  that is, for each 
child, for each tooth receiving a crown, for each crown's lifetime, the file has one row.  
A crown begins a new lifetime each time it has any kind of failure (described below).  The 
9 columns are as follows:

-- ptID:  Patient ID

-- tooth:  Tooth type within patient, using the so-called Universal numbering system for 
children.  Teeth of these types were included in this dataset:

      D:  Upper right lateral incisor
      E:  Upper right central incisor
      F:  Upper left central incisor
      G:  Upper left lateral incisor

-- crownID:  A sequential numbering of crowns (teeth) in the dataset

-- lifetime:  For the purpose of this dataset, a "crown" could have more than one 
lifetime, if a physical crown had a repairable failure and was repaired, or if a physical 
crown had a non-repairable failure and was replaced.  Although the physical crown may have 
been replaced, for the purpose of this dataset, a "crown" -- a specific tooth in a 
specific child -- simply had more than one lifetime.  This data item is the sequence 
number of lifetimes for this tooth in this child.

-- status:  Indicates how the lifetime ended, with these codes:  1: censored with the 
crown intact and in place, 2: tooth exfoliated with the crown intact and in place, 3: 
tooth extracted or pulpal infection, 4: crown re-cemented or replaced, 5: crown fractured 
or chipped, 6: fracture or chip repaired.  Status 1, 2 were considered successes, status 
3-6 were considered failures.  Status 3, 4 were considered "catastrophic" failures, status 
5,6 were considered "simple" failures, i.e., repairable.  

-- event:  This is "0" if status is 1 or 2 and "1" otherwise.

-- end:  This is time in days from when the crowns were placed, to the "end" of the 
lifetime.  The starting time is "1" (instead of the more obvious "0") to avoid having zero 
times.  

-- N_crowns:  The number of crowns the child had (1, 2, 3, or 4).

-- crown type:  The type of crown, 1, 3, or 4 (labelled I, III, IV respectively in the 
book).