Textbook of Usability

Divided attention describes attempts to attend to multiple tasks or information sources at the same time. Contrary to popular belief, true simultaneous multitasking is largely a myth for cognitively demanding tasks — they compete for the same central processing resources.

What people call multitasking is typically rapid task switching. Each switch incurs two costs:

• Time cost: 200–500 ms per switch, as the mental context of the previous task is suppressed and the new task's context is loaded into working memory
• Accuracy cost: increased errors because of incomplete context loading

The psychological refractory period (PRP) is the delay in responding to a second task when it arrives shortly after the first — evidence of a central response-selection bottleneck that cannot handle two decisions at once.

Design implications are profound, especially in healthcare where interruptions are constant:

• Assume users will be interrupted; preserve task state automatically
• Provide visual breadcrumbs of completed and remaining steps so users can resume after interruption
• Avoid designs that demand simultaneous tracking of multiple data streams
• A Westbrook et al. (2010) study found that each interruption during medication preparation increased the error rate by 12.7%

The best response to divided attention is not to train users to multitask better but to design so that task switching is supported and its costs minimised.