Productivity Principle no. 81
A single focus is infinitely more productive than a split one.
Much like decreasing the diameter of a pipe will increase the strength of the water flowing out of it, the ability to think and do is optimized when focus is concentrated. Trying to focus on two things at once will diminish the results by much more than half. "Multitasking" only works when all but one of the "tasks" is on automatic, such as driving home and wondering when you got there who actually drove! Rapid refocusing (which is what really happens in knowledge-work environments – from email to phone to interruptions, etc.) does not hinder productivity, as long as there is a clean break from one task to the next, and you are not retracing steps. Work is diminished when the focus is split, or when refocusing requires having to repeat the reframing of context and content for the next task (as in rereading sentences you've already read to find your place again).
So, keep that in mind when you jump from task to task.