One of the main problems of standards (e.g., DIN 66234, ISO 9241) in the context of usability of software quality is, that they can not be measured in product features. We developed and validated 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 between 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. The empirical outcomes of four comparison studies with six different interfaces are used to validate our quantitative usability metrics.
