Data from a clinical trial of two drugs for the treatment of
chronic asthma

   id: patient identifier
    F: PEF (peak expiratory flow rate) after treatment with Formoterol
    S: PEF (peak expiratory flow rate) after treatment with Salbutamol

The study-design was a two-period crossover. Patients 1 to 7 received 
Formoterol in the first time-period, Salbutamol in the second. Patients 
8 to 13 received Salbutamol in the first time-period, Formoterol in 
the second.