Systems, whether well designed or poorly designed, are mixtures of styles. Interface styles are used in different portions of the system as the designers feel it is appropriate. For example, some popular spreadsheets combine a direct view of the results with menus and sophisticated macro-programming facilities, which are presented in editable command languages. Limits of Categories At a gross level, these descriptions of interface style may be useful despite the problems which arise when studied at a more detailed level. We do not believe that the problems arise from the definition of this particular set of categories, but from the limits of this sort of categorical analysis.
Categorization is a simple and primary form of definition, based on the similarity between objects.
As such it is the basis of many powerful philosophical systems, particularly those derived from ancient Greek philosophy. Categorization has its limitations. It just doesn't seem to be the case that we can offer adequate definitions of things like chairs. Shneiderman points out that a number of factors affect the usability of a given class of system. In our own research we have found command, menu, and iconic systems at all levels of usability and acceptance.