There's ... enormous variation in different people in software development. Sometimes one person may be able to accomplish something in three weeks that somebody else would take a year doing. But that other person might be able to do a different thing in three weeks that the first person would have taken a year to do.
Not only are there enormous differences in productivity, but there are enormous differences in personal skill sets and in personal aptitude for certain things. And the work we're doing is just very weird and not easily controlled in an experiment. To do controlled experiments is almost impossible.
There's not a whole lot of data out there. The only evidence there seems to be is that it's very easy to find orders of 10 differences in productivity, and nobody knows why, and being faster or slower doesn't necessarily mean you do a better or worse job.
-- From "A Conversation with Joel Spolsky" in ACM Queue.