## 2 Research Methodology
### Threats to Validity  
The credibility of the reported results may be affected by the several threats to validity [33; 35]:  
* **Descriptive validity**. This threat is related to the extent to which the observations are described accurately and objectively. With the aim of reducing this threat, a data collection form was designed. This form supported the recording of data to help objectify the data extraction process and it may always be reviewed by each reviewer.
* **Theoretical validity**. It is determined by our ability to capture what we intend to. In this case, biases in the selection and extraction of the data play an important role.
* _Study selection bias._ This threat mainly concerns the reliability of the selection process that was followed to identify and select the relevant papers available in the literature on the topic under study. This selection relies on the papers returned by the automatic search performed in the three digital libraries previously mentioned. A large number of papers related to our were obtained. To mitigate this threat, each researcher ranked each study among three relevant levels according to the information provided by the abstract and even by the content of the paper.
* _Data extraction bias._ This threat concerns with the extraction of the correct data to address the MQs of this mapping study. Although this step involves human judgment, the threat cannot be removed at all. This issue was mitigated by creating a form that was filled in by the four researchers for each paper selected. A validation step was then conducted to ensure the correctness of the data extracted. The disagreements were resolved by means of discussions between the researchers. On the other hand, the quality assessment of the obtained studies was not carried out, as it is not mandatory in SMSs [34].  
\begin{table}
\begin{tabular}{l l c} \hline \hline
**Data Item** & **Value** & **MQ** \\ \hline Study ID & Number & \\ \hline Year & Year of publication & \\ \hline Author Name & Names of the authors & \\ \hline Publication Title & Name of the journal or conference in which the article & \\  & was published & \\ \hline Relevance & Manual classification ranging from A (high relevance) to C & \\  & (low relevance) & \\ \hline Place & Remote/R.centre & MQ1 \\ \hline Communication & Synchronous/Asynchronous & MQ2 \\ \hline Devices & Kinect, Wii note, Leap Motion, Physical controller, & MQ3 \\  & Physical sensors & \\ \hline Interaction paradigm & Virtual Reality/Augmented Reality/Desktop/Independent & MQ4 \\ \hline Interaction style & Natural User Interface (air-based)/Physical controller/Independent & MQ5 \\ \hline Method & Yes/No & MQ6 \\ \hline Patient Adaptation & Yes/No & MQ7 \\ \hline Automatic Game Generation & Yes/No & MQ8 \\ \hline Gamification & Yes/No & MQ9 \\ \hline Game Techniques & Basic/Standard/Advanced & MQ10 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Data extraction form.  
* **Generalizability**. This threat refers to the extent to which the results can be generalized in other groups (external) or within a group (internal). This threat was mitigated by selecting a wide range of studies related to different contexts.
* **Interpretive validity**. This threat is related to the researcher bias. This threat has been mitigated by involving the four researchers in the generation of the results.
* **Repeatability**. This threat is based on the detailed reporting of the research process. This threat has been mitigated by describing the followed systematic mapping process, including the actions taken to reduce possible threats to validity.