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The iPhone pedometer isn't always accurate, study finds

Jan. 10, 2018—Do you use an iPhone to count steps toward a fitness goal? If so, you may be walking even more than your phone is telling you.

Researchers have reported that the iPhone's built-in step-counter lost track of more than a thousand paces in an average day.

Accuracy measured

The study involved 33 people and was conducted in two parts. First the researchers manually counted the steps that participants walked on a treadmill. In their left hand, each participant carried both their personal iPhone and a standard iPhone SE provided by the lab.

Both personal and standard model iPhones measured steps least accurately when the participant was walking at a very slow pace. But at speeds above 5 kilometers (about 3 miles) per hour, the phones underestimated steps by less than 5 percent (which is considered acceptable for a pedometer).

Next each participant carried their iPhone and also wore a separate accelerometer (a kind of motion sensor) as they went about their usual day. Over three days, on average the iPhone tallied about 21.5 percent fewer steps than the other device. That works out to an average of about 1,340 steps a day for all participants.

User-wear error

To be fair to the iPhone, some participants forgot to take their phones on bathroom or water breaks, which accounts for some missed steps. And how much an iPhone might err could depend on how fast you're walking. As the lab study showed, the phones underestimated more treadmill steps at slow speeds.

If you want to use your iPhone's step counter to inspire more movement, there's no reason not to, according to the researchers. You just have to have it with you at all times, a study author said.

The researchers wanted to see how well iPhones counted steps because smartphones could be used to gather data for other studies. If so, accuracy is important.

The study was published in the Journal of Sports Sciences.

No harm done

What if your smartphone underestimates your steps? It could be an error in your favor. If it says you walked 10,000 steps in a day—a common fitness goal—you likely walked at least that many, if not more.

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