Fitness trackers.
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While fitness trackers may help monitor a healthy lifestyle, wearers shouldn’t rely on them for everything.

In a study published in the Journal of Personalized Medicine, researchers at the Stanford University School of Medicine found that most fitness monitors are terrible at measuring how many calories the wearer burns.

“People are basing life decisions on the data provided by these devices,” Euan Ashley, lead author on the study and professor of cardiovascular medicine, of genetics and of biomedical data science at Stanford, said in a statement.

Although most fitness monitors can measure heart rate fairly accurately (with an error margin of less than 5 percent), the researchers found that monitors were much less accurate on counting calories. The seven devices tested (which included the Apple Watch, Basis Peak, Fitbit Surge, Microsoft Band, Mio Alpha 2, PulseOn and the Samsung Gear S2) had an error margin of, on average, 27 percent. The least accurate device was 93 percent off while measuring how many calories the wearer had burned.

The amount by which the devices were inaccurate also depended on the wearer’s skin color and body mass index (BMI).

“My take on this is that it's very hard to train an algorithm that would be accurate across a wide variety of people because energy expenditure is variable based on someone's fitness level, height and weight, etc,” Anna Shcherbina, also a lead author on the study, said in a statement.

So people who wear fitness devices while walking, running or working out should take that data with a grain of salt.