Accuracy in smartphone and wearable devices is an important factor in their usability for medical purposes. However, a study published in the Journal of Sports Sciences found the pedometer built into the iPhone missed 1,340 steps when compared to an accelerometer worn on the waist.

Smartphones, health apps and wearable activity trackers have become new tools in patient monitoring and data collection, but evidence into their accuracy is lacking. In this study, researchers investigated the accuracy of a smartphone-based pedometer compared to a purpose-built accelerometer.

"In order to make accurate conclusions, we as researchers need to know that the data is actually representative of real behavior," said Mark Duncan, the study's lead author. "That has major impacts in terms of patient care, and in terms of developing new and better research in the field."

The study enrolled 33 participants who underwent two tests, one set in a laboratory and the other in a regular living environment. The first phase provided patients with two iPhones, a personal phone and one provided by the lab, and collected data while participants walked on a treadmill for 60 seconds at various speeds. The second phase required participants to wear an accelerometer on their waist for a full day while also having their steps recorded on an iPhone.

Results of the first phase showed the personal iPhone underestimated the number of steps by 9.4 percent at the slowest speed while the lab given iPhone underestimated at 7.6 percent. Additionally, researchers noted the phones were more accurate in their step counting the faster the walking speed. The second phases of results showed the iPhone again underestimating an average of 21.5 percent of daily steps. However, many participants reported leaving their smartphones at home, resulting in a low daily step count.

"For people who are already tracking their steps, they can rest assured that if their phone says they're getting the recommended 10,000 steps in a day, they are probably getting at least that many, and they are working toward better health," said Duncan. "From a public health point of view, it's better that it underestimates than overestimates."