## Appendix A
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Interaction** & **Data Type** & **Description** \\
**Parameter** & _float_ & Time spent in a certain game level (s) \\ \hline _times-played_ & _int_ & Number of game plays \\ \hline _score_ & _float_ & The ratio of correct and incorrect answers/the final score \\ \hline _used-help_ & _int_ & Number of help requests \\ \hline _skipped_ & _boolean_ & Flag indicating whether the task was skipped \\ \hline _screen-size_ & _string_ & Device screen size (width x height) \\ \hline _ambient-intensity_ & _float_ & The value of the light intensity in the environment \\ \hline _ambient-noise_ & _float_ & The value of ambient noise in the environment \\ \hline _target-distance_ & _float_ & Distance between the user (device) and the target \\ \hline _condition-value_ & _float/int_ & The value indicating the condition to be achieved, e.g., time (s) or size \\ \hline _object-distance_ & _float_ & Distance between the positions of two virtual objects \\ \hline _time-for-interaction_ & _float_ & Time required for the user to interact (s) \\ \hline _progress_ & _float_ & Percentage of progress (value between 0 and 1), e.g., progress in the game or interacting with the object \\ \hline _interaction-type, number_ & _string:int_ & The type of interaction that was used and the number of times the interaction was detected \\ \hline _rotate-direction_ & _string_ & The direction in which the user rotates the virtual object (_left_, _right_, _none_) \\ \hline _attempt-number_ & _int_ & Number of attempts \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Interaction parameters specified for tracking events in the context of accessibility. After the specification of interaction parameters, it is necessary to define the data structure of previously specified events, i.e., key interaction points. Since the model for tracking user interactions in AR-based serious games proposed in this work is based on the interaction model defined in [58] (see more in Section 3.1.3), it is necessary to map the event attributes and previously defined interaction parameters to xAPI statements. To recall, the interaction event is defined with the following attributes: timestamp, user identifier, action, target, and optional value. For example, if we are tracking the event when the user launched the game, the event trigger for this interaction point is _the user launched a serious game on the device_. In this example, the _action_ attribute corresponds to the verb _launched_, while the _target_ corresponds to the object type presenting _a serious game_. Possible _optional values_ in this event could be various contextual information, such as information about the device and/or time of the day. The interaction event attributes of a serious game are mapped to xAPI statement attributes as shown in Table A3. \begin{table}
\begin{tabular}{c c} \hline \hline
**Interaction Event Attribute** & **xAPI Statement Attribute** \\ \hline user id & actor \\ \hline action & verb \\ \hline target & object \\ \hline optional value & result, context \\ \hline timestamp & timestamp \\ \hline \hline \end{tabular}
\end{table} 
Table A3: Mapping of interaction event attributes to xAPI statement attributes (adapted from [58]). The xAPI standard identifies verb and object attributes with a unique Internationalized Resource Identifier (IRI), as well as the extensions of statements within the result and context attributes. In addition, we bring possibilities of how different interaction event attributes can be mapped to follow the xAPI standard and to cover different aspects of AR-based serious games in the context of accessibility. Table A4 shows the mapping of xAPI verbs (represented by IRIs) which can be used to cover actions of interaction events with an example of a corresponding key interaction point in an AR-based serious game earlier specified.