TABLE 1.

Factors that may reduce the level of involvement in the communication loop (i.e., reduce ecological validity), examples of these factors, and corresponding metrics that may be used as a proxy to quantify the level of involvement (with references, if available)

	Factor	Example	Metric	References
1	Visual stimuli differ from those experienced in the real-life condition under test	Low video reproduction quality; use of animated characters with low quality of appearance	Similarity of head and eye movement behavior to real-life behavior; Questionnaires/subjective scaling	Hendrikse et al. (2018)
2	Acoustic stimuli differ from those experienced in the real-life condition under test	Low sound field reproduction quality; static (nonmoving) sound sources	Quantitative measures of the function of a (simulated) hearing device in comparison to its functioning in the real-life condition under test	Grimm et al. (2016), Hendrikse et al. (2020)
			Questionnaires/subjective scaling	Hendrikse et al. (2018)
			Head- and eye-movement behavior	To be investigated
3	Unrealistic behavior of conversation partners in the (simulated) experiment	Animated characters do not move heads and eyes or do not show facial expressions	Head- and eye-movement behavior	To be investigated
4	Lack of interactivity of the experimental paradigm	The paradigm does not promote a closed-loop communication similar to the real-life condition under test	Subjective scaling of the involvement; gaze behavior or facial expression	To be investigated

*Note that head and eye movements were used both as a metric and as experimental factors by Hendrikse et al. (2018): head and eye movements of communication partners in the (simulated) experiment were used as an experimental factor that was found to influence the metric, that is, the head and eye movement of the subjects in the experiment.

†The sound events and levels experienced by each subject during the walks were not controlled. This means that the sound levels in the field and in the lab were the same on average, but not for each subject individually. Individual dosimetry would have been required to make individual adjustments.

‡Because HA amplification was the same in both conditions and no noise reduction was employed, this finding suggests that the background noise level may have been somewhat higher in the field than in the lab and may have partially masked the sounds from the bicycles.