Go read Peopleware. You will find described very carefully set up coding comparisons that routinely found a factor of 10 productivity difference between different experienced programmers on the same task, and also a discussion of what organizational factors lead to those productivity differences.
There is other research on the topic as well. For instance http://www.computer.org/portal/web/buildyourcareer/fa035?utm... cites "individual differences" research that found a 30 fold difference between mediocre programmers and the top programmers. That article is supposed to be a distillation of http://www.amazon.com/Facts-Fallacies-Software-Engineering-R... so I'd look there if you want citations into how that research was done and what exactly they found.