Grape et al. (Integrative Physiological and Behavioral Science, 2003)
investigated the possible beneficial effects of singing on well-being.
Their study subjects were 8 amateur and professional singers.  On each
subject, the level of cortisol was measured before and after a singing
lesson.  The alternative hypothesis of interest was whether cortisol level 
increases during singing lessons.  

The dataset includes two variables:

    before  -- person's cortisol level before the singing lesson
    after   -- person's cortisol level after the singing lesson