As stated before is the main mission of the science of MMI an empirical sound measurement of important properties of a certain human machine interaction. The main technical term in use today to describe the wanted properties is the term 'Usability'. The term as such has no clear meaning. If we want to use this term within an argument where the usability experts have to convince the design experts to change their prototypes on account of usability arguments the usability people have to provide a clear 'meaning' of the term 'usability' which is understandable, clear, empirical sound and reproducible.
This situation is the usual situation in empirical sciences in general! If you want to set up a sound empirical theory (including a formal model) then you have to introduce some practical measurements to get empirical data, and based on these data you can set up some formal structure as a formal model.
In the case of the science of MMI it is a common practice to replace
the general term of 'Usability' by a set of more concrete terms which
all together shall represent the --then compound-- term 'Usability'
|Figure: Meaning of the Term 'Usability'
One very common set of sub terms to the term 'Usability' is shown in the figure (cf. Lauesen, 2005:9). But such a set of sub terms is still without a clear meaning! Somehow they can mean nearly everything. Used within an argument without any further explanations will cause lots of emotions but no real insights.
The only way to associate these sub terms with an empirical meaning is the way to couple each sub term with a finite set of measurements. Thus the sub term 'Ease of Learning' could be coupled with a method to measure 'Task Time'. In this case one could try to define the meaning of the term 'Ease of Learning' with regard to the time a user will need to learn the task. And similar in all the other cases.
In practice this is not as simple as it may appear in the first moment. We will have a look tho this in the following sections.
(For a good introductory example of the design and practice of a usability measurement see (Lauesen, 2005:Chapters 1 + 13).START