I don't have the book ready here, and it's been a long time since I last read it, but wasn't that the productivity for specific, limited and well-specified task

I don't have the book ready here, and it's been a long time since I last read it, but wasn't that the productivity for specific, limited and well-specified tasks? How about the productivity of tasks that only a subset of developers would be able to solve (which don't have to be that hard, like a bug-free binary search, which according to Bentley[1], eluded 90% of professional programmers)? How do you quantify the difference in performance for that? [1] Programming Pearls. Jon Bentley (Column 4)