Do one thing at a time; don't reply to messages during a meeting

Moderate value
Evidence B
Saves time
Free
Takes minutes
Some willpower

In plain words

When you switch tasks, you react more slowly and make more mistakes. People who often have several screens going at once have not trained themselves to do several things at once; they actually do worse on task-switching tests.

Cost: Free. Holding back from replying to messages in a meeting takes a little willpower.
What you get: After switching tasks, reactions were clearly slower and errors more likely. This part comes from a review of several studies. Another lab study had college students as subjects. People who often use several kinds of media at once (heavy media multitaskers) were more easily pulled off course by irrelevant things and irrelevant memories. They actually did worse on a task-switching test.
Notes: How much extra effort switching back and forth costs varies a lot with the work being done, so no single figure is given here. The Ophir study compares people at one point in time, so it cannot tell whether “multitasking makes attention worse” or “people with poor attention multitask more.”
Evidence grade: B : Supported by research but without a precise number, or based on small or single studies.
Sources: Monsell (2003). Task switching. Trends in Cognitive Sciences. https://doi.org/10.1016/S1364-6613(03)00028-7
Ophir, Nass & Wagner (2009). Cognitive control in media multitaskers. PNAS. https://doi.org/10.1073/pnas.0903620106

Translated from the Chinese original. Original book, section 3, tip 7. Content may be out of date and rules differ by place. When you actually deal with an agency, a doctor or a court, follow what local officials, doctors and lawyers tell you. Disclaimer