Usability Levels - Three Ways to Care for Usability
SAP: All three levels of usability work are needed for developing usable applications. Technical prerequisites and style guides create the fundament on which usable applications can be built, but do not guarantee them. Only the "user-and-task" level ensures that a development team will really end up with a usable application that serves the needs of its users. (2001-04-22)
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