Textbook of Usability

The split-attention effect occurs when related information required to understand or complete a task is presented in spatially or temporally separated locations, forcing the user to mentally integrate the sources. Each integration consumes limited working memory resources and increases extraneous cognitive load.

The effect was identified by John Sweller and colleagues as part of Cognitive Load Theory. Classic experiments show that learners presented with a diagram and separate text explanation perform worse than learners presented with the same explanation integrated directly into the diagram as annotations.

Interface examples of split attention:

• A form field's error message displayed in a sidebar instead of next to the field
• A diagram explained by text on a different screen
• A medication dose calculation where the patient's weight is on a paper chart and the drug concentration is on the computer
• Validation messages that appear at the top of a long form rather than beside the offending field
• Reference data that the user must remember while entering values elsewhere

Mitigations:

• Co-locate related information that must be used together
• Integrated displays combine multiple data sources into a single visual
• Contextual help appears near the element it explains
• Autocomplete and preview bring relevant information to the point of use

The split-attention effect is a specific, measurable way that bad layout raises cognitive load even when each individual element is well designed.