It used to be assumed that differences among hospitals
    or doctors in a particular specialty were generally 
    insignificant. If you plotted a graph showing the 
    results of all the centers treating cystic fibrosis—or 
    any other disease, for that matter—people expected that 
    the curve would look something like a shark fin, with 
    most places clustered around the very best outcomes.
    But the evidence has begun to indicate otherwise. What
    you tend to find is a bell curve: a handful of team
    with disturbingly poor outcomes for their patients, a
    handful with remarkably good results, and a great
    undistinguished middle.