Textbook of Usability

Visual search is the task of locating a target in a visual field containing distractors — finding a specific button in a toolbar, a patient's name in a list, or a file in a folder. Visual search time depends on how easily the target can be distinguished from its surroundings.

Two fundamentally different search modes exist:

• Parallel search (or pop-out): when the target differs from distractors in a single pre-attentive feature (colour, size, orientation), it is detected essentially instantaneously regardless of how many distractors there are. A red item among blue items pops out the same whether there are 5 or 50 blue items.
• Serial search: when the target is defined by a combination of features or is visually similar to distractors, the user must examine items one at a time. Search time increases roughly linearly with the number of items.

In practice, interface search is often a mix: users may quickly narrow to a region using pre-attentive cues, then serially scan within that region.

Design implications:

• Make frequently used targets visually distinctive on a single feature
• Group related items spatially so users can search within categories
• Order items by frequency of use or by logical category so common targets are found first
• Avoid long flat lists of visually similar items — they force worst-case serial search