> How do you measure your productivity?

The same way everyone measures productivity: subjectively. With software development, measuring productivity objectively is very much an unsolved problem (which is one of the major problems with studies on productivity of different floor plans, incidentally).

> What do you attribute to your increase in productivity in an open floor plan?

1. Generally feeling happier, more energized, and more focused (I'm an extrovert). When I work alone I find myself getting restless, irritable, and distracted.

2. Collaboration. Catching mistakes, soliciting creative ideas, getting information when I don't know something. But really #1 is a much bigger factor for me.