Personal baseline methods for wellness data
Wearable measures differ widely between people and devices. A personal baseline compares each person with their own recent normal, which is how ATS-1 reports change.
Population norms versus personal baselines
A population norm asks how one person compares with others. A personal baseline asks how today compares with that person's own recent days. For general wellness, the FDA's 2026 policy describes products that display values, ranges, trends and baselines in the context of sleep, activity, stress or recovery (FDA, General Wellness: Policy for Low Risk Devices, reissued 6 January 2026). ATS-1 reports change against a personal baseline and shows no population thresholds.
Choosing the window
| Choice | Trade-off | ATS-1 default |
|---|---|---|
| Window length | Longer is steadier; shorter follows real change sooner | 28 days, rolling |
| Warm-up | Too few days gives an unstable baseline | At least 14 valid days before any index is shown |
| Center and spread | Mean and standard deviation react to one unusual night | Median and MAD × 1.4826 |
Missing data
Gaps are normal: devices get charged, forgotten or replaced. Count valid days rather than calendar days, report the count alongside every value, and do not fill gaps with invented values. ATS-1 outputs carry their valid-day count.
Device and firmware changes
A new device or algorithm can shift readings without any change in the person. Record the device model, firmware and algorithm version on every record, and treat a change as a reason to rebuild the baseline rather than as a change in the person.
Reporting change honestly
- Describe change relative to the person's own baseline, never as good or bad
- Show the uncertainty and the number of valid days
- Avoid thresholds and alerts that imply a health judgment
- Expect false alarms when real changes are rare; try the false-alarm calculator
What this guide does not claim
ATS-1 is for research and general wellness only. It does not diagnose, treat, prevent or predict any disease, including any mental health condition. For questions about your own health, speak to a healthcare professional.
Questions
Why 28 days?
It is ATS-1's design choice: long enough to be stable, short enough to follow seasonal and lifestyle change. It is marked as a design choice in the specification, not a sourced value.
Sources
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How to build a personal baseline from wearable data: window length, warm-up, robust statistics, missing data, device changes and honest reporting.