I mentionned before the fact that multitaskers could not be more efficient than single minded people because there was a cognitive cost associated with having to upload their memory of the previous tasks. In a remarkable study, Gloria Mark goes farther and notes:
What we found is that the average amount of time that people spent on any single event before being interrupted or before switching was about three minutes. Actually, three minutes and five seconds, on average. That does not include formal meetings, because we figured if they were in a formal meeting, they were prisoners at the meeting, right? They couldn't leave or switch activities. So we didn't count that. Then we looked at [use of] devices, working on a PC, the desk phone, using any kind of paper document, using a cell phone. We found the average amount of time that people spent working on a device before switching was 2 minutes and 11 seconds.
But we called it a "working sphere" because "project" has a limited connotation, and a working sphere is a broader idea. Anything where there's a common goal, there's a certain group of people involved with it, there are certain resources attached to it, it has its own time framework and its own deadline, is a working sphere
So even when we took out what we call "non-significant" interruptions, we find that people still worked 12 minutes and 18 seconds in a working sphere before switching.
But there are also internal interruptions; for whatever reason, people interrupt themselves of their own volition and switch to something else. And what fascinates me is that people interrupted themselves almost as much as they were interrupted by external sources. They interrupted themselves about 44% of the time. The rest of the interruptions were from external sources.
GMJ: How long does it take to get back to work after an interruption?
Mark: There's good news and bad news. To have a uniform comparison, we looked at all work that was interrupted and resumed on the same day. The good news is that most interrupted work was resumed on the same day -- 81.9 percent -- and it was resumed, on average, in 23 minutes and 15 seconds, which I guess is not so long.
But the bad news is, when you're interrupted, you don't immediately go back to the task you were doing before you were interrupted. There are about two intervening tasks before you go back to your original task, so it takes more effort to reorient back to the original task. Also, interruptions change the physical environment. For example, someone has asked you for information and you have opened new windows on your desktop, or people have given you papers that are now arranged on your desk. So often the physical layout of your environment has changed, and it's harder to reconstruct where you were. So there's a cognitive cost to an interruption.
The designers of a task manager for a small business could learn a lot from these results. For instance, in my case, the application we developed at my workplace was aimed at handling the communication load of about 60 people with many different schedules, projects and from different functional groups. After some time, we noticed certain behaviors. First, we found out that if the application was not used by everybody uniformly, sooner rather than later, an assymetry of the information stream helped people who were least likely to contribute/use the system. In effect, cooperation between users was less effective over time. The second finding was a little more subtle. The application was written in PHP but did not use AJAX. Every time the page would load with new information, it took the new page about 10 seconds to refresh. In light of the study by Mark, it seems that this is enough of a burden that this cost of "waiting" for the refresh would have people drift into other applications and other "working spheres". For participants, the time spent in updating knowledge in the application felt like "feeding the beast" as opposed to being part of a normal cognitive process.