Wednesday, February 11, 2015

Inaccurate fitness monitors

Lara Lewington at the BBC's Click has an article on the website about fitness/activity monitors that's quite interesting for those inclined towards gadgets. How long it will be available I'm not sure, but currently you'll find it here. Thanks to Adam for pointing to the article.

In simple terms, the article points out that if you take four of the most prominent players in the market, you will get significant discrepancies in the data they present to you about your activity levels and related data. I have to say that I don't find that particularly unexpected, and it certainly fits with my experience of my Polar Loop (which wasn't among the four tested). I guess the object lesson in all of this goes back to something I learned doing physics at school. We were taught the difference between precision and accuracy. As I recall, and it is a rather long time ago, the difference is that you can have very precise measurements that are inaccurate and conversely you can have accurate results that are imprecise. Sounds crazy I know, but we are talking about the mysteries of physics here. In common usage we might use accurate and precise to describe the same thing, but in science they mean different things.

So what does this mean for the activity tracker you're planning on strapping to your wrist? The most important implication is that the data you get has to be understood against the way the technology works, the kind of sensor it has and the maths it uses to convert what it measures into the data you get to read off the device. I've already blogged about the differences between a simple pedometer and my Loop and how the Loop converts body movements into steps. Even my pedometer doesn't necessarily measures steps exactly.

So why would you spend your hard earned cash on an activity monitor that can't tell you either accurately or precisely why you've been up to? I'm not sure the technology exists to do that, but that doesn't mean a monitor is a bad thing. The reason to buy one of these gadgets lies in a quote from one of the companies making the devices has to say.

Managing director of Fitbit Europe Gareth Jones advises people to simply be aware of the trend.
"Rather than get down to the half step or the next calorie is to look at the trend in their step pattern," he says,
"Are they increasing the number of steps in their day all week? Are they increasing the calorie burn day to day, week to week? Because it's that trend that's going to make you healthier."

It's pretty obvious that a device worn on your wrist is unlikely to give you a proper measure of steps taken, but it will indicate how active you've been. It's these trends that help you make choices about what you need to do to improve your health. So buy a gadget if you want, or get someone to buy one for you (mine was a Christmas present) and use it to motivate yourself to move. It's fun to see what yo've been doing, how far you've walked or run, how long you've spent sitting down. But don't get hung up on the raw data, look at the trends. Look at the bigger picture.

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