In each of two centres, eligible patients were randomly assigned 
    to active treatment or placebo. During the treatment, the respiratory
    status (categorised poor or good) was determined at each
    of four, monthly visits. The trial recruited 111 participants
    (54 in the active group, 57 in the placebo group) and there were
    no missing data for either the responses or the covariates. The
    question of interest is to assess whether the treatment is effective
    and to estimate its effect.
Note that the data are in long form, i.e, repeated measurments are
    stored as additional rows in the data frame.