# Why a personal baseline beats population averages

Canonical: https://getearlysignal.app/en/guides/why-personal-baseline-beats-averages

> Wellness scores that compare you to everyone hide the early personal shifts that matter.

## In short

Population averages for HRV, resting heart rate, and overnight temperature are so wide that early personal change disappears. Early Signal builds robust baselines from your own nights—median and MAD—so status reflects unusual patterns for you, not a generic normal range.

Last reviewed 2026-09-20

## The problem with normal ranges

Two healthy people can have very different usual HRV or RHR. Flagging someone for being outside a population band creates noise; missing a personal shift creates false calm.

## Median and MAD in plain language

The median is the middle night. MAD measures how spread out typical nights are. Robust z-scores ask: how unusual is today compared with your own day-to-day spread? Outliers move a mean; they barely move a median.

## Learning period

Until enough valid HRV and RHR nights exist, the app stays in Learning and withholds Off from usual alerts. Importing existing Apple Health history often unlocks a baseline on day one.

## Guide FAQ

### How many nights do I need?

The engine targets a multi-week robust baseline of valid nights. Exact thresholds live in the on-device scoring config; the UI explains progress while Learning.

## Sources

Public documentation cited as context—not as clinical validation of Early Signal.

- [MedlinePlus — Heart rate variability](https://medlineplus.gov/ency/article/007200.htm)
- [Apple — Track your sleep on Apple Watch](https://support.apple.com/guide/watch/track-your-sleep-apd830528336/watchos)

## Privacy · Terms · Science

GetEarlySignal is a wellness tracking and rest-day support tool. It does not provide medical diagnoses, treatment plans, clinical assessments, or healthcare advice. The app does not replace professional medical care or emergency healthcare services.

- [Privacy](https://getearlysignal.app/en/privacy)
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