- obs
 
a numeric vector. Number of observations.
    
- id
 
a numeric vector. Id of subject.
    
- visit
 
a numeric vector. Timings of the visits in years.
    
- smoke
 
a numeric vector code. 0: non-smoker, 1: smoker.
    
- age
 
a numeric vector. Age of the patient at the start of the trial.
    
- cd4
 
a numeric vector. CD4 percentage at the current visit.
    
- cd4.prev
 
a numeric vector. CD4 level at the preceding visit.
    
- precd4
 
a numeric vector. Post-infection CD4 percentage.
    
- lt
 
a numeric vector. Gives the starting time for the
      time-intervals.
    
- rt
 
a numeric vector. Gives the stopping time for the
      time-interval.