On Wednesday the series "Cutting the Fat TODAY" continued with a Rossen Reports investigation putting the most popular fitness trackers to the test.
First TODAY national investigative correspondent Jeff Rossen tested two free fitness apps for smartphones: Apple Health for iPhones and Google Fit for Android devices.
Carrying both devices in the same hand, Rossen walked around 30 Rock for more than an hour. Theoretically, the two devices should have counted the same number of steps. But when Rossen compared the counts on the two devices, they differed by more than 100 steps.
Both Apple and Google told NBC News they had no comment.
Next, Rossen donned three fitness-tracking devices on the same wrist — a Jawbone UP3, a Garmin vivosmart HR, and a Fitbit Charge HR — and walked around New York City for more than an hour. When he compared their step counts afterward, the three counts varied by hundreds of steps.
Then Rossen went to the performance lab at New York's Hospital for Special Surgery. Still wearing the three fitness trackers on one wrist, he walked 500 steps. Afterward, one of the three devices came close to the exact step count, measuring 502 steps; the other two registered 520 and 559.
The hospital's technology also measured how many calories Rossen had burned during those 500 steps. None of the three devices was even close to that count; one said he had burned nearly quadruple that number.
The companies told NBC News that the trackers are designed to motivate and encourage healthy lifestyles. Garmin added that "the position on the wrist" mattered "if multiple devices were worn at the same time" and that it also mattered whether the device was worn on your "dominant hand."
Experts say not to expect absolute precision from your fitness tracker. But it's still a good idea to get out there and walk.