The purpose of an investigation by Schwartz et al. was to quantify the
effects of cigarette smoking on standard measures of lung function in patients
with idiopathic pulmonary fibrosis (a lung disease).  Among the measurements
taken were percent predicted residual volume.  This dataset contains
two variables:

   smokhist -- N = never smoked, F = former smoker, C = current smoker
   pprv     -- percent predicted residual volume

There are 21 N's, 44 F's, and 7 C's.