I was once told by a professor at a prestigious university (paraphrase):
In the average software development workplace, the best programmer is 30 times faster than the worst programmer. So if it takes the office's best programmer one day to write some particular code, it will take that same office's worst programmer an entire month to write the same code. These two programmers will often be making roughly the same salary.
Is this true? Are there any references or studies to corroborate this?