Interactive Multimodal Applications (IMAs) support communication with the user through different modalities such as voice and gesture. They can greatly improve human-computer interaction, because they can be more intuitive, natural, efficient, and robust. Multimodality brings an intuitive, natural affinity between the machine and the user, such as in virtual reality mobile application. Efficiency is obtained when the user can use equivalent modalities for the same tasks while robustness can result from the integration of redundant or complementary inputs [35].

The CARE properties (Complementarity, Assignment, Redundancy and Equivalence) can be used as a measure to assess the usability of the multimodal interaction. Equivalence and assignment represent the availability and, respectively, the absence of choice between multiple modalities for performing a task while complementarity and redundancy express relationships between modalities. The flexibility and robustness of interactive multimodal applications result in an increasing complexity of the design, development and testing. Therefore, ensuring their correctness requires thorough validation [34].