AUDIE CORNISH, HOST:
Companies that make fitness trackers advertise great things in a small package. A sleek wristband with a small digital screen can measure how many steps you take, how well you sleep, how many calories you burn, even how stressed out you are. But are fitness trackers accurate? A study out today tries to answer that question, as NPR's Patti Neighmond reports.
PATTI NEIGHMOND, BYLINE: It's happening more every day, patients coming to see cardiologist Euan Ashley armed with reams of data gathered from their fitness tracking device.
EUAN ASHLEY: They may be bringing in heart rate monitoring data or step count data or calorie estimation data. And they're saying, this is something that I want you to use in giving me lifestyle advice about how to avoid cardiovascular disease.
NEIGHMOND: The problem was Ashley just didn't know how reliable the data were. He works at Stanford Hospital and clinics in California. He and colleagues decided to study seven of the most popular devices and compare them to the gold standard doctors use. The first measure - heart rate. The fitness trackers were compared to findings from an electrocardiogram.
ASHLEY: We were pleasantly surprised to find that it was actually pretty accurate.
NEIGHMOND: Most devices most of the time were off by only about 5 percent. But when it came to measuring calories burned, the findings were way off when compared to a sophisticated system used in doctors' offices.
ASHLEY: There was a significant degree of inaccuracy with the calorie estimations. In fact, it ranged from 20 percent up to 90 percent.
NEIGHMOND: The worst-performing device was off by 93 percent, making its findings more like fiction than fact, says obesity and exercise researcher Dr. Tim Church.
TIM CHURCH: People are checking these inaccurate calorie counts and they think they've earned a muffin or they've earned some ice cream. And they're sabotaging their weight loss program.
NEIGHMOND: Church points to a study last year which found participants in a weight loss program who also wore fitness trackers actually lost less weight than participants who didn't wear the trackers.
CHURCH: It's an instance of no information's probably better than having bad information.
NEIGHMOND: The Stanford study published in the Journal of Personalized Medicine was relatively small, 29 men and 31 women. Participants were a diverse group, says Ashley, and there were some other interesting findings.
ASHLEY: In certain groups of people - for example, those with darker skin or males or those who are a little bit larger - the error actually was greater. And so for some of the people for whom it would matter the most, maybe those who are trying to lose weight, the error was actually greater.
NEIGHMOND: Ashley doesn't know why this is but speculates it could be that companies use a fairly narrow group of people when designing the equations they use to measure heart rate and calories burned. Makers of two devices, Fitbit and PulseOn, say they remain confident in the performance of the trackers in measuring both heart rate and calories burned. In a statement, PulseOn takes issue with the study's methodology. The other device makers didn't get back to us with a comment. Patti Neighmond, NPR News.
(SOUNDBITE OF THE LIMINANAS SONG, "TROUBLE IN MIND")
NPR transcripts are created on a rush deadline by an NPR contractor. This text may not be in its final form and may be updated or revised in the future. Accuracy and availability may vary. The authoritative record of NPR’s programming is the audio record.