Summary
Interference effect is a phenomenon that occurs when mental processing is made slower due to contradicting interpretations. The textbook recommends avoiding designs that create conflicting mental processes to lessen the burden on the user. A few examples of interference effects include:
Stroop Interference: an irrelevant aspect of a stimulus triggers a mental process that interferes with processes involving a relevant aspect of the stimulus.
Proactive Interference: existing memories interfere with learning
Retroactive Interference: learning interferes with existing memories
Original Example
Reading the words aloud is easier than naming their colors. The mental process for reading is more practiced and automatic and, therefore, interferes with the mental process of naming the colors.
Online Example
The conflicting message interferes with the mental process of driving.
Real Example
Naming the shapes out loud is easier than reading the titles
Source
Lidwell, William, et al. Universal Principles of Design: 125 Ways to Enhance Usability, Influence Perception, Increase Appeal, Make Better Design Decisions, and Teach through Design. Rockport Publishers, 2003.