Switching Condition --
for allowing a paradigm switch during query formulation. The problem needs to be investigated both from the system's and from the cognitive viewpoint.

Expressive Power --
achievable in the different modes may not be the same. Form-based and diagrammatic paradigms often provide the same expressive power as the relational algebra but VR only allows selection and retrieval of objects. For interaction through different media using speech or stylus-drawn gesture, such modes have the further disadvantage of ambiguous input. A solution is changing the interaction mode so that in the new mode a certain operation is no longer ambiguous.