QuitForecast / The manual / Health and baselines
Health and baselines
The forecast says your body recovers. The Health tab is where your own Apple Health data gets to agree, against your own before.
The five metrics
Resting heart rate, heart rate variability, cardio fitness, respiratory rate and blood oxygen. Each is charted against your own pre-quit baseline, with what published research expects underneath.
Each card charts your own daily averages from Apple Health, with your pre-quit baseline drawn beside them. Beneath the chart sits what published research expects that metric to do after quitting, with the study named. Your chart is allowed to disagree with the research; the app never edits your line to match the story.
Heart rate variability is the early mover worth knowing about: nicotine suppresses it, and studies show it climbing within days to weeks of stopping. The readings need an Apple Watch or a similar sensor feeding Apple Health; without one the tab says so and waits.
The baseline
Your own average from before the quit, drawn only when enough pre-quit days exist to mean something. A baseline from two readings is noise, so the app waits rather than guesses.
The baseline is your average from the fourteen days before your quit date. Those are fourteen days you did not know you were recording, which is what makes the before-and-after honest. Backdate your quit accurately at setup so the line lands inside the history your watch actually holds.
If those days hold too few readings to mean anything, no baseline is drawn and the chart stands alone. A dotted line invented from two data points would be the chart lying with statistics.
What never leaves the phone
Health data is read on your device and never transmitted anywhere. It is never backed up by this app, and never quoted by any surface that leaves the phone. That is a promise the code holds by test.
The app reads your Health data fresh each time the tab opens and keeps no copy. No series on your phone, none in the iCloud backup, nothing on any server. Access is read-only, and the permission sheet asks for exactly the five metrics and nothing else.
No surface that leaves your phone quotes a Health number: not the backup, not the board, not feedback. The app's own test suite enforces this, which is what the manual means when it says the promise is held by code.