The key to universal usability is recognizing the diversity of user population and user needs. There is no "average" user on whom a system should be based. Although in some cases it is possible to accommodate technology variety and individual differences in one system, multi-layer designs are the most promising approach to achieving universal usability. That is, when a single design cannot accommodate a large fraction of the user population, multiple versions or adjustment controls should be available to users. For example, a novice user can be provided with only a few options; after gaining confidence and experience, the user can choose to progress to higher levels of tasks and the accompanying interface.