iPhones Are Really Bad at Tracking Your Steps
Apple's Health app underestimates your daily step count by up to 21 percent.
If you’re using your iPhone as a pedometer, we have good news and bad news. The bad news is that it may not be giving you an accurate step count. The good news is that it’s likely underestimated: You’re probably walking more than you realize.
Researchers at the University of British Columbia conducted a study comparing the pedometer of the iPhone’s built-in Health app with a purpose-built pedometer worn at the waist and found the former undercounted by about 1,340 steps during a user’s typical day. If your phone says you’re getting the recommended 10,000 steps a day, you’re probably getting at least that many.
The study had two parts, one in the laboratory and one under real-world conditions. For the laboratory test, the 33 participants carried two iPhones at the same time; researchers compared results from their personal iPhones to that of shared laboratory iPhone to look for discrepancies against the number of steps researchers counted manually. Participants walked on a treadmill for 60 seconds at various speeds while having their steps counted manually. At the slowest speed or 2.5 kilometers per hour (or 1.5 miles per hour), personal iPhones undercounted steps by 9.4 percent, with the shared iPhone undercounted by 7.6 percent. It’s common for pedometers to function better at a faster pace, and at walking speeds of 5 kph and above (3.1 mph and higher), the phones were off by less than five percent. That’s considered a reasonable margin of error for a pedometer.
The second part of the study, the real-world test, had participants wear research-grade accelerometers at their waists for the whole day, then compared those readings to the step count produced by their iPhones. Over three days, the iPhone trailed the accelerometer data by an average of 21.5 percent—or 1,340 steps per day.
The phone may not be entirely to blame, however. Some participants admitted leaving their phones behind when they went to the bathroom or took short walks to get water. And daily life involves a lot of slow-paced walking, which the above lab test had shown hurts accuracy. One takeaway here might be that if you want better data, you may want to jog from your desk to the water cooler. Kidding! You could keep your phone in your pocket when you walk around the office but then you’d have to remember not to sit on it and also try not to drop it in the toilet.
Overall, the UBC study suggests that while it’s tempting to want to harness all that easily produced data, researchers should understand its limitations before placing too much faith in it. Your average iPhone owner, on the other hand, probably just wants a general sense of how they’re doing—and maybe a digital badge to brag about among friends.
“The accelerometer in the iPhone actually does a pretty good job when tested under lab conditions," senior author Guy Faulkner said in a release. "You just have to have it on you at all times.” Or you can take its count as an estimate, and not worry about whether you’re losing 1,340 somewhere along the way. Most of us underestimate how much we eat anyway.
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