The main problems of standards (e.g., ISO 9241) in the context of usability of software quality are, that they can not measure all relevant product features in a task independent way. We present a new approach to measure user interface quality in a quantitative way. First, we developed a concept to describe user interfaces on a granularity level, that is detailed enough to preserve important interface characteristics, and is general enough to cover most of known interface types. We distinguish

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... etween different types of 'interaction points'. With these kinds of interaction points we can describe several types of interfaces (CUI: command, menu, form-fill-in; GUI: desktop, direct manipulation, multimedia, etc.). We carried out two different comparative usability studies to validate our quantitative measures. The results of one other published comparative usability study can be predicted. Results of six different interfaces are presented and discussed. One of the most important result is that the dialog flexibility must exceed a threshold of 15--measured with two of our metrics--to increase significantly the usability.