The performance of doctors follows the normal distribution. I think that extrapolating to programmers is not a huge stretch.

    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.

http://www.newyorker.com/archive/2004/12/06/041206fa_fact?cu...

The difference between doctors and programmers is that programmers get to build/leverage tools that are abstractions of other tools, hence you can have orders-of-magnitude differences in productivity between programmers who use the best tools and those who don't. I assume that there's not that much variation between the way two different doctors carry out the same task like there is with programmers.

Actually, doctors do have order-of-magnitude tools for improving certain metrics, such as recovery and infection rates: http://www.newyorker.com/reporting/2007/12/10/071210fa_fact_...