Resting physiological baselines differ across people; absolute values cannot be compared across users

What it is

Quiet sitting is not one number. One person’s tonic skin conductance can be several times another’s; resting heart rate can differ by a dozen beats; HRV norms span even more. Absolute microsiemens or absolute bpm are almost never the same ruler across users. Cross-person comparison has to go through relative change or a within-person percentile, or “above threshold equals stress” permanently flags people whose intercept is high.

Why it happens

Peripheral physiology is already written into DC at rest by anatomy (sweat-gland density, heart size), drugs, fitness, and chronic state. Tonic EDA especially: humidity, site, and gland function set the level; phasic responses track events more. A population cut (heart rate >100) means opposite things in an athlete and a febrile patient. Statistically this is a random effect: each person has an intercept; the quantity of interest is a slope or a deviation from that intercept. Norms are barely usable when the distribution is tight and the use is screening rather than fine adaptation.

Studying it

Collect a no-task rest per person, then elicit, and report between-person baseline variance relative to within-person evoked amplitude. Factor: whether individual z-scores or percentiles are used. Outcomes: accuracy gap between a cross-person classifier and a within-person one. If a public dataset already subtracted baseline and a paper still sells an absolute threshold, preprocessing has cancelled the claim. Age, drugs, and site must be strata, not post-hoc stories.

Where it stops holding

Some clinical cuts (fever, extreme bradycardia) have population-medical meaning; that is a diagnostic frame, not a “stress score” for UI adaptation. Even one user’s two hands, or two seasons, can differ, so “individual” is not iron. Absolute error of optical heart rate is device-dependent; a cross-user comparison is first a device bias. Whether a BCI rest rhythm exists at all is itself an individual difference that cannot share one absolute-power gate.

Applying it

• Adaptation and alerts should be relative to that user’s rest or recent distribution; do not treat a factory norm as personal state.
• Run an explicit baseline take on first use and tell the user it is not a task.
• If a leaderboard uses a physiological score, switch to within-person progress so high-baseline people are not shamed.
• Verify by applying A’s absolute threshold to B’s rest; a high mislabel rate means the product still compares absolute numbers across people.

Related

• Same group: C9.10.2 After calibration sets an individual baseline, affect, fatigue, and time drift the baseline itself · C9.10.3 Long-term use needs periodic recalibration; a one-shot calibration does not last · C9.10.4 Unrecognized baseline drift is misread as a state change
• Adjacent: C9.03 Heart Rate and Electrodermal Activity · C9.02 Brain-Computer Interfaces
• Search: individual baseline · tonic SCL · within-subject z-score