# Affective Stroking: Design Thermal Mid-Air Tactile for Assisting People in Stress Regulation
## 2 Materials and Methods
### Measurements  
According to previous research [13], due to individual differences, comparing the mean data between groups during the relaxation phase does not effectively reveal the  
Figure 9: Experiment Procedure.  
actual impact of the design intervention. Instead, comparing the mean changes in metrics (relaxation phase--stress induction phase) between the SG and CG is a more accurate way to assess the true effect of our design intervention. We used a subjective scale and two physiological indicators as follows:  
* State-Trait Anxiety Inventory (STAI);  
For this experiment, which focused on short-term changes in anxiety rather than long-term anxiety issues at the individual level, we used the part of \"state\" of the State-Trait Anxiety Inventory (STAI-S) [71] to collect subjective data from participants at the end of each experimental phase. The questionnaire consisted of 20 items and used a 4-point scale for responses, ranging from 1 (\"not at all\" or \"almost never\") to 4 (\"very much\" or \"almost always\"), based on the participant's current state.  
* Root Mean Square of Successive Differences (RMSSD);  
RMSSD [73; 74; 75] is a commonly used metric for assessing heart rate variability (HRV). It measures the differences in time intervals between successive heartbeats (RR intervals), reflecting instantaneous changes in heart rate and the ability to self-regulate. In HRV analysis, RMSSD is primarily used to evaluate the activity level of the parasympathetic nervous system, which plays a crucial role in heart rate regulation. An increase in RMSSD indicates heightened parasympathetic activity, often observed during recovery from high-intensity exercise or stressful environments [74; 75]. Conversely, a decrease in RMSSD suggests reduced parasympathetic activity, which is associated with higher levels of stress [74; 75].
* Low Frequency/High Frequency (LF/HF);  
In HRV analysis, LF and HF [76; 77] are commonly used frequency domain indicators. The LF/HF ratio is frequently used to assess the balance between sympathetic and parasympathetic nervous system activity. A higher LF/HF ratio may indicate predominant sympathetic activity, which is typically associated with higher stress levels [76; 77]. Conversely, a lower LF/HF ratio may suggest a dominant parasympathetic activity, which is generally linked to lower stress levels and better relaxation [76; 77].  
We calculated HRV metrics from the cleaned RR intervals for each 5 min segment of our experiment [47]. The processing software was Kubios 4.1.1. This 5 min window is selected as it is the widely accepted minimum duration for deriving neurophysiologically significant HRV metric [74].  
* Interview.  
In our interviews, we addressed eight questions across three parts (Appendix A). The first part (Q1, Q2) focused on the sensory experience and related associations of the computer-mediated stroking itself (with emphasis on the perception of stroking, temperature sensation, comfort, and pleasantness). The second part (Q3-Q6) involved the participants' perceptions during the relaxation phase of the experiment and the impact on individual stress regulation (whether or not it was effective). The third part (Q7, Q8) discussed potential improvements in design features and future usage scenarios. We audio-recorded interviews with each participant. The data were transcribed following the methodology of qualitative content analysis [78]. Two researchers independently transcribed and analyzed the recordings line by line, extracting descriptions relevant to the themes. They named similar events and situations, categorized them, and conducted frequency counts, including the number of participants under each description, to highlight the significance of each theme.