Wearable Metrics Are Trend Signals

Smartwatch sensors (accelerometer, gyroscope, PPG, GPS) record fairly accurate raw data, but proprietary algorithms transform that data into higher‑level health metrics that become progressively less reliable the farther they are from the sensor — e.g., step count and heart rate are more trustworthy than calories burned, sleep stages, or hydration estimates. Users and clinicians should therefore treat many wearable outputs as relative trend indicators rather than absolute clinical measurements, and switching brands can change the numbers because the algorithms differ. — This reframes debates about consumer health devices toward algorithmic provenance and regulatory claims, affecting medical use, consumer protection, and how public health messages should incorporate wearable data.