As user interfaces are assembled out of sets of components (e.g. pop-up menus, radio buttons and list boxes), suitable theories about user-system interaction are needed to guide design and evaluation in creating usable products this way. The creation and deployment of components are allocated in different processes. Therefore, usability evaluation of a component in the creation process would be more efficient than testing the usability of the component each time it is deployed in an application. Usability evaluation in the deployment process is not necessary if the usability of the entire application only depends on the usability of the individual components. The Layered Protocol Theory (LPT), a multi-layered user-system interaction theory, claims the ability to evaluate the usability of components empirically. The theory also claims the independence of component’s usability of other components in the user interface. This study tests these claims and tries to answer the following question: Is usability compositional? The main research question is broken up into two underlying questions:

1. Whether and how the usability of components can be tested empirically.

2. Whether and how the usability of components can be affected by other components.

The research was conducted in a series of laboratory experiments in which users had to perform tasks with prototypes of various user interfaces. An explorative first experiment was conducted using a fictitious user interface, which resulted in a hypothesis for a behavioural component-specific usability measure. It indicates the users’ effort to control their perception of a component. Based on this, a component-based usability testing framework was proposed and tested in a second experiment. A third and fourth experiment focused on the factors inconsistency and mental effort as factors that could cause the usability of a component to be affected by other components. 

The first main conclusion drawn from this research is that LPT indeed provides a basis for empirical evaluation of the user interface on different layers, which strengthens the claim that this theory can be applied in the area of user-system interaction. The second main conclusion is that the usability of a component can be affected by other components. This means that the usability of the entire product can not be predicted solely on the usability of the individual components. Inconsistency and mental effort are factors that allow one component to reduce the users’ ability to control another component.