Label definition in wrist-based ambulatory stress monitoring: reference state, label width and matching direction

This is the authors' abstract. We don't add key points for this paper.

Wrist-worn sensors provide continuous physiological time series, but using them to monitor stress requires a label built from three choices: the person's normal reference state against which the signal is contrasted, the temporal width assigned to a self-report, and the direction in which that report is matched to the signal. We quantified how much these choices change the measured association, using a dataset in which the objective context and the perceived stress were recorded independently over the same periods. Twenty-six students wore an Empatica EmbracePlus for several weeks, providing electrodermal activity, blood volume pulse, skin temperature and acceleration, alongside a dated calendar of academic events and ecological momentary assessments. Electrodermal activity was decomposed into tonic and phasic components, pulse-to-pulse intervals were corrected for ectopic and missed beats, and modality-specific quality criteria were applied before any contrast. The same participants completed a controlled session used to check that the chain detects a physiological response under aligned conditions. Contrasts were computed at the participant level as Cliff's delta with bootstrap confidence intervals, and differences between contrasts were tested directly. Examination periods were associated with lower cardiac variability relative to baseline, whereas high perceived-stress reports were not. Each of the three labelling choices substantially changed the measured effect size, by up to a factor of 1.7 on the same recordings. Label definition should therefore be reported, varied, and treated as a parameter of the processing pipeline, on the same footing as filtering or artefact rejection.