Nielsen Norman GroupEvidence-Based User Experience Research, Training, and Consulting
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What users believe they know about a UI strongly impacts how they use it. Mismatched mental models are common, especially with designs that try something new.
Parallel design is a project model for usability engineering where multiple designers independently of each other design suggested user interfaces. These interfaces are then merged to a unified design that can be further refined through additional iterative design. In a case study of a screen-based telephone interface, measured usability when going from version 1 to version 2 was improved by 18% when using traditional iterative design and by 70% when using parallel design. In the case study, parallel design was about 73% more expensive than iterative design, so it cannot be recommended for all development projects, but it seems a promising usability engineering method for speeding up time-to-market. Nielsen, J., and Faber, J. M. (1996). Improving System Usability Through Parallel Design. IEEE Computer, 29, 2 (February), 29-35.
In 4 case studies, the median usability improvement was 165% from the first to the last iteration, and the median improvement per iteration was 38%. Iterating through at least 3 versions of a UI design is recommended, since some usability metrics may decrease in some versions if a redesign has focused on improving other parameters. Nielsen, J. (1993). Iterative user interface design. IEEE Computer 26, 11 (November), 32-41.