For me multi-tasking usually results in lower quality.
Actually "focus on multiple tasks at once" seems like a contradiction in terms to me since, if your attention is divided between multiple things, then it is not focused, by definition.
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I do sometimes feel a bit of a rush of believing I'm accomplishing a lot of things done at the same time by multi-tasking. But most of the time, I have had to repay the benefit by correcting for mistakes or having to completely re-do what I did earlier.
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I suspect extroverts may be more likely to say they multi-task well. Especially, the "Get-Things-Going" types (ENFP, ESFP, ENTP, ESFJ).
I think multi-tasking may be effective for "executive" job positions, that rely on other individuals to take care of the actually quality of work-- because there isn't "quality" per say in the act of delegation. It is more about getting people on tasks and getting them working, than really doing some sort of high-quality assignment of tasks.
Of course, longer-term placement of people in positions would require more care.
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Interestingly, from a computational perspective, this is similar to the problem computer architects and software engineers have in making use of multiple threads of execution.
On the one hand, you get the hardware utilization up, so that execution units, etc. aren't as idle as serial execution.
On the other hand, the programs get more complicated--you have to pay for the overhead of parallelizing the tasks, which includes synchronization between tasks and maintaining coherence of the data being shared.
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Anyone read
CrazyBusy?
I suspect substitute has because of the story about using a rotary telephone.