Although smartwatches can offer helpful health information, many of the numbers they display aren’t as accurate as users may believe.
According to a new study by researchers at the University of Michigan School of Kinesiology and Hope College, many of the numbers that smartwatches display are algorithmic approximations rather than precise measurements, and some outcomes are more accurate than others. Often, these approximations merely rely on sensor signals, proprietary mathematical algorithms, and assumptions about the wearer.
“The most important takeaway is that not all smartwatch metrics should be interpreted the same way,” said Adam Lepley, assistant professor at the University of Michigan School of Kinesiology. “Some outputs are relatively close to what the device’s sensor actually detects, while many others are estimates generated by combining sensor signals with proprietary algorithms, user characteristics and other assumptions.
“People shouldn’t take these metrics at face value. In many cases, these devices are better suited to tracking trends over time, rather than as precise laboratory measurements.”
Key takeaways from the study of smartwatches
The researchers shared three key takeaways from their study, which was published in the journal Sensors.
1. Smartwatch data are most useful for tracking changes within the same person over time. A consistent shift in resting heart rate, sleep or activity may be more meaningful than a single unusual reading.
2. Resting and steady state heart rate, step count and outdoor pace are generally more reliable than complex estimates such as calories burned, sleep stages, body composition, hydration and recovery.
3. Accuracy can also be affected by movement, watch fit, temperature, sweat, skin tone, tattoos and body composition. Results may not be comparable across brands because manufacturers use different sensors, definitions and algorithms.
According to the study, nearly one in three Americans say they use wearable technology regularly. Given how much daily decision-making depends on these devices, it is important for people to understand the actual accuracy of the figures that smartwatches provide.
Not much has changed in the past six years
Unfortunately, it doesn’t appear that, despite their increasingly complex metrics, the validity and reliability of smartwatches and the data they provide have not become significantly more accurate since they were studied six years ago.
In a 2020 study, researchers concluded, “Commercial wearable devices are accurate for measuring steps and heart rate in laboratory-based settings, but this varies by the manufacturer and device type. Devices are constantly being upgraded and redesigned to new models, suggesting the need for more current reviews and research.”
Based on this new study, much of what the researchers determined in 2020 still applies.