Study Highlights Inaccuracy of Many Fitness Trackers

A study published in the Journal of the American Medical Association looked at a number of fitness trackers and smartphones to see how accurate they are at counting steps. A number of tests were carried out on treadmills, with participants walking 500 or 1,500 steps. The devices were compared with the actual steps that the participants took.

The test showed that many fitness trackers are inaccurate, something that I recently discussed, but looking at the results raises a number of questions.

Jama fitness tracker test

First, why would different apps on the iPhone show different results? All apps that track steps do so using the built-in motion co-processor.

Second, they give a step count for the Nike Fuelband. But that device doesn’t track steps; it uses “Nike+ Fuel” as a metric.

Finally, why did they only test these devices on treadmills? That, in my tests, is where they are the most accurate. Fitness trackers need to be tested during everyday activities, because any worn on the wrist track steps when you make certain arm motions.

I note that the study confirms what I have seen (among the devices I’ve tested): that the FitBit One (, Amazon UK) is the most accurate at counting steps. It may not be the sexiest device, but it certainly does what it claims.

Some people claim that the accuracy of these devices isn’t that important; that it’s more important to look at them as recording trends. This is true, if all you do is check how many steps you count in order to be more active. But many people use these together with calorie counters, and the lack of reliability of the step count – and other activity tracking – means that any calculations of calories burned is wrong, ofter by a very large percentage.

I think it’s a shame that so many of these devices are sold that are grossly inaccurate at the one metric that they claim to measure. Consumers should demand more than just estimates.

Fitness Trackers Need to Provide More than Just Data

Fitness trackers are good at counting things: steps you walked, calories burned, active minutes, how long you slept, and so on. While the accuracy of these devices can be dubious, a tracker can at least tell you how active you are compared to other days–well, assuming you keep wearing it. According to the Journal of the American Medical Association, more than half of the people who purchase fitness trackers stop using them, and one-third stop in less than 6 months.

What fitness trackers and their apps aren’t good at–yet–is providing real insights. While looking at your fitness data can help motivate you to maintain a level of activity, data alone probably doesn’t tell you that much that you don’t already know. They don’t help you get healthier or more active. You have to do that yourself.

For fitness trackers and other health-related devices to be useful, they need to go one step further and provide real, actionable advice.

Read the rest of the article on Macworld.

Shouldn’t Fitness Trackers Be Accurate?

314xSphvUfLI’ve written about fitness trackers here several times. I currently have a Fitbit Charge, I’ve reviewed the Fitbit Flex, Fitbit One and Jawbone UP24, and I tried the Nike+ FuelBand.

One problem with these devices – aside from comfort, which is not always a given – is their accuracy. I’m surprised that most websites that review these devices don’t even discuss accuracy, when it’s not that hard to compare different fitness trackers, and determine how accurate they are. It’s almost as though reviewers expect them to be inaccurate, and don’t think that accuracy is important. (One notable exception is this Mashable review.)

I’ve been using the Fitbit Charge for a few weeks, and I’m surprised at how inaccurate it is. Fitbit, on their website, tout the accuracy of the device, saying:

“Charge has been tested extensively against our clip-based devices like the Fitbit One and Fitbit Zip. That said, because Charge is specifically designed for your wrist, if you move your body a lot and not your arms (or vice versa), you may get a slight difference in activity than you would see on your clip-based trackers. Additionally, since you’re more likely to wear Charge 24/7, you may count a few more steps.

“This is no different than any wrist-based tracker on the market. For most people, there may be no difference at all between clip and wrist based trackers or it may be within a few percentage points difference. That said, if you have a lifestyle where you move your hands a lot such as playing the drums every day, you may see a few extra steps on your Charge because we do want to give you credit for this activity.”

Yet it’s horrible. Compared to the Fitbit One, the Charge records 30-50% more steps, and, therefore, longer distance and more calories burned (and its “active minutes” is pretty wonky too). It even records some steps while I sleep. Granted, the Fitbit Flex records lots of steps when you drive, so I guess this is an improvement. And the Jawbone UP24 doesn’t record steps when you walk on a treadmill, so all the Fitbit devices win on that front.

Here’s another example of the Fitbit Charge getting it wrong. I went out to mail some letters today; the mailbox is 400m away. First, when putting on and tying my shoes, the Charge recorded 20 steps. Then, I set the Fitbit app to record my walk. When you do this, it uses the GPS in the iPhone (or other phone) to record a map of your walk, providing you with distance, time and pace per mile or kilometer. The app recorded this correctly, and said I had walked 794 steps; the Charge recorded 943 steps.

Most people don’t compare the accuracy of different devices, so they never know how inaccurate their devices may be. Precise accuracy isn’t essential; as I said in my review of three of these devices:

“Fitness trackers are motivators. While, on the surface, they claim to record data about your activity, the real reason people buy them is to motivate themselves to be more active. None of them are perfectly accurate, and they all have drawbacks.”

Thinking about this made me wonder how companies think they can sell these devices with such glaring inaccuracy. I understand the technical hurdles in counting steps using a wrist-worn device; but what Fitbit says, above, is that it’s not only possible, but accurate. They say that the charge “may count a few more steps,” but they’re disingenuous if they really think that it’s only a few steps.

Fitbit says:

“Fitbit is dedicated to developing the most accurate activity trackers on the market. Our team performed multiple internal studies to rigorously test the accuracy of our trackers. Through our testing, we have confirmed that our trackers are some of the most accurate wireless tracking devices.”

And, on another page:

“We’ve tuned the accuracy of the Fitbit tracker step counting functionality over hundreds of tests with multiple body types. All Fitbit trackers should be 95-97% accurate for step counting when worn as recommended.”

So the problem is one of a marketing message that is simply not true; where I grew up, we called that “lying.” 95-97% accurate would be great, if it were true. Assuming that the Fitbit One is the most accurate of the devices I’ve tested, because it’s worn on your belt, like a standard pedometer, none of the other devices are anywhere near that accuracy. The Nike+ FuelBand cops out, most likely because the company realized that accuracy is simply not possible with a wrist-worn device; they came up with “Nike+ Fuel,” a points-based system that has no relationship to steps. In a way, they’re the most honest; they don’t pretend to be accurate. However, Nike still claims that their software can count calories using an algorithm based on the energy you expend when you move. That’s pretty vague, and deflects the issue.

The fitness tracker sector is expanding rapidly, and the one thing companies should do is ensure that their devices are accurate. I think these devices do have a future, but only if we can take them seriously. For now, we simply cannot.

Wearable Review: Fitbit Charge Activity Tracker

Update: Having discovered serious problems with the accuracy of this device, I’ve updated my review on January 2, 2015. I leave my original review intact below.

In my original review of the Fitbit Charge, which you can read below, I said, “it’s as accurate as the Fitbit One.” I’ve come to find that this is not the case.

I originally bought the Fitbit Charge when it was released in the UK. On December 26, Amazon UK had a one-day sale, offering the device at 25% off. I ordered one, and returned the original device. While the original device was fairly accurate in my testing, this new device is terribly inaccurate.

Here are some examples. One day, I tested the devices by wearing both of them. I went about my usual business, and I walked on my treadmill for 30 minutes. Both devices recorded nearly the same number of steps on the treadmill (2,048 steps for the Charge, and 1,997 for the One), but for the rest of the day, the numbers diverged greatly. Near the end of the day, when the Fitbit Charge was at 5,000 steps, the One was almost exactly at 4,000 steps. These convenient numbers make it very easy to calculate the discrepancy between the two devices. If you take away the 2,000 steps on the treadmill, where the devices nearly matched, the Charge recorded 3,000 steps, and the one 2,000. In other words, the Charge is recording 50% more steps than the One. (I’ve noticed that the Charge records some steps when I’m asleep; a half-dozen here, a dozen there, adding up, some nights, to 50 steps or so. And, no, I don’t sleepwalk.)

In subsequent days, I saw similar numbers. The Charge generally records from 30-50% more steps. What this means is that the distance it tells you you’ve walked is 30-50% higher than what you actually walked (or ran), and the calories that it says you burn are much higher than what you have really burned. (Not 30-50%, because the Fitbit also takes into account your basal metabolic rate, or the number of calories your body burns if you do nothing.)

But it goes much further than that. Today, I walked on my treadmill for exactly 30 minutes. When I started, there were no active minutes recorded; when I finished, there were 35 active minutes recorded.

2015 01 02 12 22 31     2015 01 02 12 22 28

Looking back, I see that this is the case almost every day. Yesterday morning, it recorded 6 active minutes when I went downstairs and made breakfast. And the day after the above 35-active-minute workout, the Charge recorded 27 active minutes for the same 30-minute walk on my treadmill.

Not only is the device reporting incorrect information, but the Fitbit website is also presenting incorrect past statistics. Looking back, I see that my step counts are grossly over-exaggerated. In March of this year, it shows me as having gone over 20,000 steps several times. I was using the Fitbit One, which seems very accurate, but here it looks as though the website algorithm – which is probably what takes the raw data from the devices, and converts it into numbers – is way off base. There’s no way of knowing if this is what’s causing the discrepancies between the two devices, but if it were the overall algorithm, then both devices would report incorrect numbers now, and that’s not the case. When I look back at active minutes, I also see this. I’m not a very active person, working at home in a sedentary job, and there’s no way that my activity was anywhere near what is shown in the screenshot below.

2015 01 01 20 50 13

An article on the Fitbit website discusses the accuracy of the device. It says:

“Charge has been tested extensively against our clip-based devices like the Fitbit One and Fitbit Zip. That said, because Charge is specifically designed for your wrist, if you move your body a lot and not your arms (or vice versa), you may get a slight difference in activity than you would see on your clip-based trackers. Additionally, since you’re more likely to wear Charge 24/7, you may count a few more steps.

“This is no different than any wrist-based tracker on the market. For most people, there may be no difference at all between clip and wrist based trackers or it may be within a few percentage points difference. That said, if you have a lifestyle where you move your hands a lot such as playing the drums every day, you may see a few extra steps on your Charge because we do want to give you credit for this activity.”

If it were just “a few extra steps,” I would understand. But this is much more than a few. And that doesn’t explain why 30 active minutes count as 35; that’s something that you simply don’t get wrong because the device is worn on the wrist as opposed to on the belt.

With this in mind, I strongly recommend avoiding the Fitbit Charge, if you seek a device that is accurate. While the first model I had was quite accurate, the fact that the second is so far off suggests that there is a serious problem with the quality of these devices. It’s pretty much a crap shoot as to whether the device you get is accurate. If you do buy one, I strongly suggest you check its accuracy. You can do this by, for example, counting your steps per minute on a fixed-speed treadmill, or even outdoors on a flat surface, then extrapolating to get the total steps for the amount of time you walk. You can also check the distance by walking around a track, if you have access to one. The way the Fitbit calculates distance is by simply multiplying the number of steps you take by your stride, and you can set that in the settings on the Fitbit website, in the Personal Info section. You can measure your stride by taking a number of steps along a distance that you can measure, such as in a room whose dimensions you know. (I suggest that you measure this over at least ten steps.)

I contacted Fitbit’s PR firm, who sent me another Charge. I wore two charges on the same wrist for two days, and they both gave more or less the same numbers (one was a tad higher than the other). So it’s not just a single bad unit, it’s the Charge in general that’s inaccurate. I’ll stick with the Fitbit One.

Original review:

I’ve been using the Fitbit One fitness tracker for about two years. It’s easy to use, since it clips on your belt and you can forget about it. But I like the idea of the fitness tracker, and have tried several others, such as the Nike+ Fuelband, and the the Fitbit Flex and Jawbone Up 24. I found that all of these trackers – with the exception of the Fitbit One, were inaccurate, and many were uncomfortable.

314xSphvUfL.jpgFitbit has released a new fitness tracker, the $130 or £99 Fitbit Charge. (, Amazon UK) This device is similar to the Fitbit Flex, but with a larger OLED display that can show the time, steps taken, distance travelled and more. Unlike the Flex, however, the Charge is very accurate. When I tried the Flex for a few days, I found its numbers diverged widely from those of the Fitbit One, especially when walking on a treadmill, or when driving in a car; it counted about 100 steps during a 15-minute drive, for example. A waist-worn tracker is probably the most reliable in counting steps, and the One is a good benchmark. Fitbit seems to have greatly improved the accuracy here, making the Charge seem much more accurate than its predecessor, and than the Jawbone Up 24, which was also very inaccurate.

2014-12-11 09.53.05.pngThe Fitbit Charge records your steps, floors you climb (using an altimeter; 10 ft = 1 floor), your “active minutes,” and your sleep. Using steps, it gives you a rough idea of the distance you’ve travelled. It also calculates the number of calories you have burned, based on your activity; you can log food as well, to determine how much you’ve eaten, and set a goal of burning more calories than you consume each day. You can record your weight, either by entering it manually, or by syncing with Fitbit’s Aria scale. (, Amazon UK) (I use one of these, and it’s easy to use, and syncs to your Fitbit account over Wi-Fi.)

You can also record walks or runs, using the Fitbit iOS app. This records your distance, splits by mile or kilometer, and, using the iPhone’s GPS, even saves a map tracking your route. You initiate a recording by using the Track Exercise section of the iOS app, and choose Walk, Run or Hike. You can also start this by pressing and holding the button on the side of the Charge for a few seconds.

Fitbit’s iOS app is the best of the fitness apps I’ve seen, as compared to those of Nike, Jawbone and Withings. It can display lots of information, and lets you set goals. When you reach your goal – which can be steps, distance, calories burned, active minutes, floors climbed or a calorie deficit – the Charge vibrates to tell you. And the Fitbit website has a graphical dashboard that shows all of your data.

Fitbit web

All this can seem a bit finicky; you may not want to view all of this data, and you can choose what to display, and in what order, in the Fitbit app. There are also a number of customization options for the device itself, which you can set from the app:

2014-12-11 09.53.22.png

You can see above that there is a tap gesture; this lets you choose what displays when you tap twice on the device. I found this very hard to get right. I initially wanted to double-tap the way I double-click a mouse, but that didn’t work; I had to tap very hard to get the device to display. I eventually found that you need to double-tap fairly slowly to get it to work, and I’ve gotten used to it, but figuring that out wasn’t easy, and the Fitbit website offers no help on this matter. (I just tried it again, as I was writing this article, and it took four tries to get it right; Fitbit needs to fix this.)

While much of the data the Charge records and displays is useful, I’m not convinced that the sleep data tells me much. Unlike previous Fitbit models, where you had to manually engage a sleep mode, this one detects when you go to sleep by the fact that you’re not moving. And the sleep data it offers is sketchy at best. Since it’s based on your movement, you may be awake and not sleeping while the device thinks you are asleep.

2014-12-11 10.15.41.png

The Charge also offers two useful features, but that could be better implemented. It has a vibrator that can alert you to incoming calls, and the display shows caller ID. This is great, except it only vibrates once. If you miss that vibration, and don’t hear your phone (mine is generally set to silent), it ends up being useless. It should continue vibrating until you’ve picked up the call. You can also set “silent alarms,” which vibrate – and continue – at times you set in the Fitbit iOS app. It takes too many steps to get to the alarm settings, and it would be more useful if you could set them more easily. You can set repeating alarms, which is good if you get up every morning at the same time, but they can only repeat daily. So if you want to set an alarm for every day of the week, you need to set five alarms. And not forget to turn it off on holidays or when you’re on vacation.

As for the device itself, it’s comfortable, having a soft plastic strap that is thin enough that it doesn’t get in the way when I’m typing, as the Jawbone Up 24 or Nike+ Fuelband did. It comes in two sizes; I have big bones, and the large model just barely fits. You need to leave a finger’s thickness between the band and your wrist to be able to attach the device by pressing its stainless steel clasp against the underside of the wristband, and I can just barely squeeze my finger under it. I would have liked a slightly larger model, which would be a bit looser. The Fitbit website only says there are two sizes, but the product manual says there is also an extra large size, and I’ve contacted Fitbit to find out how one can purchase that model, and if I can exchange mine. (I bought it from Amazon, so I know I can return it.) If you’re hesitant about the sizes – small is 5.5″ – 6.7″, and large is 6.3″ – 7.9″ – opt for one that’s larger than your wrist, and if you have a large wrist, contact Fitbit. (If I get more information about the extra large size, I’ll add it here.)

Fitbit has another version of this coming out in a couple of months, the Fitbit Charge HR. This will cost a bit more, and will have a heart rate sensor, to record your heartbeat continuously. I don’t really need this. And the Fitbit Surge, which will be larger, more like a Pebble watch, will also include GPS; again, not a feature I need, since my iPhone can handle that.

One note about syncing the device. There are two ways to sync: using the iOS (or Android) app, or using a dongle, provided with the device, which you connect to your computer. The former works fine; the latter hasn’t worked for me, even with the Fitbit One, for some time. Several months ago, it stopped working, and Fitbit sent me a new dongle; that worked for a while, then stopped. I have been unable to get the desktop software to sync the Charge, even though I re-installed the software. It seems to think that the Charge I have is a different one than the one linked to my account (which I’ve synced using my iPhone), and fails. I find that the desktop software is problematic, at one point, causing high CPU usage, and Fitbit support is not too helpful regarding these issues.

While I haven’t had it long, I like the Fitbit Charge. I actually find it useful to be able to check the time on my wrist; I haven’t had anything that allowed me to do that for a long time, but I’m finding that I do use it as a watch. The device is comfortable, and it’s easy to forget that I’m wearing it. It’s not the most attractive of devices, but it looks much better than, say, the massive Nike+ Fuelband, or the oddly-shaped Jawbone Up 24. And it’s as accurate as the Fitbit One. I’m still tempted by the Withings Activité, which looks like a real watch, but, for now, the Fitbit Charge is a keeper.

Fitness Tracker Review: Fitbit One, Fitbit Flex & Jawbone UP24

Fitness trackers are motivators. While, on the surface, they claim to record data about your activity, the real reason people buy them is to motivate themselves to be more active. None of them are perfectly accurate, and they all have drawbacks. Some have good hardware and mediocre software; some have excellent software and poor hardware. But, if you want this kind of device, there is certainly one that will fit your needs.

I’ve tried three fitness trackers: the Fitbit One, the Fitbit Flex, and the Jawbone Up24. I’ve had the Fitbit One for about a year and a half; I tried the other two recently. Here are my thoughts. (Note that all three of these devices sync to a smartphone by Bluetooth; the Fitbit devices also come with a USB dongle to sync to a computer. Also, each of them uses a proprietary connector to charge; it goes into a USB plug, but it’s yet another cable to worry about.)

Fitbit One

The Fitbit One (, Amazon UK) is a tiny tracker that you can either clip onto your pants or belt, or carry in your pocket. It tracks steps, and a built-in altimeter counts the number of floors you climb. At night, it can also track your sleep, both in time and quality. The latter assessment is based on how restless you are at night.

pnl1_fitbit_one.jpgAs I said, I’ve been using the Fitbit One for about a year and a half. I bought it for two reasons: to motivate me to become more active, and to lose some weight. The Fitbit software takes the data from the tracker and calculates the distance you’ve walked, and the number of calories you’ve burned each day; you can set goals and try and reach them.

I’ve been using this in conjunction with Fitbit’s Aria wi-fi scale (, Amazon UK), which records my weight and syncs to the Fitbit website, where that data integrates with data from the tracker, and displays in the Fitbit iOS app.

The Fitbit One is discreet, and doesn’t get in your way. Since you wear it on a waistband or belt – or even stick it in your pocket – you don’t have to worry about it being visible, which is not the case with the wristband trackers I’ll look at below. However, I lost the first Fitbit One I bought, when traveling; its clip wasn’t as tight as I’d like. The replacement I bought is tighter, and I’ve had it for about 15 months.

The Fitbit One is very accurate at counting steps; it’s in the best location to do so, right around your hips, where you would wear a simpler pedometer. Fitbit’s iOS app lets you watch your steps live, and I’ve tested the One walking on various types of ground and at different speeds, and it always registers steps. As for floors, however, that’s not so precise. It counts a floor if you go up or down ten feet in altitude; your floors may be more or less than that, and I’ve found that the floor data is pretty useless.

I don’t use the Fitbit One to track sleep. It comes with an elastic wristband, into which you slide the device, to wear at night. It’s uncomfortable and annoying. I tried it for a few nights, then gave up.

Overall, the Fitbit One is a good device, as its step count is extremely accurate. Fitbit’s software – both on iOS and its web-based dashboard – is useful, though relatively simple. It doesn’t offer reminders and nudges as the Jawbone UP does; it essentially offers just raw data. While I want more – the inactivity alert the Jawbone can provide would be useful to remind me to get up and move during the day – it’s good enough for now.

Fitbit Flex

The Fitbit Flex (, Amazon UK) is a wrist-worn Fitbit device. Offering the same tracking data – with the exception of floors – this device is quite limited in its ability to convey information. While, with the Fitbit One, you can press a button on the device to cycle through the day’s data, the Fitbit Flex only shows a few LEDs to tell you if you’ve reached your goal.

61YRYwYtSJL._SL1500_.jpgUnlike the Fitbit One, the Flex is very inaccurate. It records around 20% more steps than the One, and during a 15-minute drive to a grocery store, it recorded about 100 steps (tested in both directions). Since it’s worn on the wrist, it cannot be as accurate in counting steps; using Fitbit’s iOS app, which shows a live step count, I can see that it records steps when, for example, I reach my arm to a bookcase to the left of my desk to grab a book and place it on my desk.

If you wish to use it for sleep tracking, however, it is a lot easier than the Fitbit One. The idea of wrist-worn trackers is that you wear them all day, so you don’t need to change the device from your belt to your wrist. However, you do need to tap the device in a certain way to engage sleep tracking (five quick taps just below the LED display), and remember to disengage it the next morning. Last night, I slept, according to the Fitbit iOS app, from 12:29 to 8:28. Yet the software tells me I slept 7h 41mins; the math isn’t that hard, and the difference doesn’t even correspond to my “11 restless minutes.”

The Fitbit Flex comes with two wristbands, in a small and large size. I found the large to be comfortable, and not too tight. However, the clasp that holds it shut is just a piece of plastic that you push through two holes in the wristband; it’s hard to put on, and I can’t see this staying on during, say, a basketball game, or ever when you pull off certain clothes. At the cost of these devices, and given that I’ve already lost a One, I don’t trust this type of clasp.

While it’s comfortable, the Fitbit Flex’s inaccuracy makes it essentially useless. I’ll have more to say about that in my conclusion below.

Jawbone UP24

I was very attracted by the concept of the Jawbone UP24 (, Amazon UK), because, unlike the Fitbit, this device nudges you to be more active. This can be through notifications from its app, and you can set a reminder to warn you when you haven’t been active for a certain number of minutes (the wristband vibrates).

JawboneUp24-582_size_blog_post.jpgI bought the large Jawbone UP24, after measuring my wrist to be 19 cm (the large size is for 18-20 cm wrists). Surprisingly, the Jawbone is tight on my wrist, even though I don’t have fat wrists. I am big-boned, but the size I measured, following Jawbone’s instructions, should be fine with this model. It’s tight enough that the two ends of the device don’t lie flat as they should (as you can see in the photo to the left). The device should also be loose enough to allow air to flow under it, especially if you’re wearing it when active and sweating; this isn’t the case for me.

I took the Jawbone for a walk. I have a treadmill in my house, and walked for a half-hour with the Jawbone on my wrist: it recorded a total of 38 steps, compared to the Fitbit One, which recorded a bit over 2,000 steps.

I looked on the web, and saw this is a common problem with the Jawbone. It measures steps by the movements of your arms, so if your arms aren’t swinging – such as when you walk on a treadmill, or when you’re carrying something – it won’t count that activity. Apparently, it also doesn’t count steps when you walk slowly, such as in a supermarket.

I’ve seen recommendations that you should put it in your pocket when walking on a treadmill, but if you have to do that, it defeats the purpose of using this type of device. So I tried that; the same half-hour on the treadmill, and the same 2,000+ steps with the Fitbit. The Jawbone, in my pocket, recorded 260 steps.

So, an activity tracker that can’t count your activity is not very useful. Add to that the fact that it’s uncomfortable. When I’m typing, the Jawbone gets in the way. The heels of my hands rest on my desk – I touch-type – and the Jawbone is in a position where it touches the desk. I can’t put the jawbone any higher on my wrist, since there’s no extra room; when I try, I can feel that it constricts my wrist a bit. The Fitbit Flex, on the other hand, is as thick as a standard watchband, and doesn’t bother me when I type.

One more thing

I would have liked to try the Fitbit Force, the most recent Fitbit product. This is a wrist-worn device, like the Flex, but which offers more options, and a real display (it can display numbers, not just LEDs, and also shows the time of day). However, this device has been recalled, because of a large number of users who got rashes from its plastic. Also, it has a similar clasp to the Flex, and I’d probably be afraid of losing it.


What’s the point of an activity tracker if it’s not accurate? Of the three I’ve tried, the Fitbit One, because of its location, is clearly the most accurate. If you have set a goal for your activity – most people use the arbitrary round number of 10,000 steps per day – you’ll hit it a lot quicker with, say, the Fitbit Flex than the One. Which means that you won’t be as active as you want.

The software for these devices also counts calories. If you’re trying to diet with a specific calorie restriction, the incorrect step count will also give you a very skewed calorie count. For a difference of +/- 20% in steps, this leads to a similar difference in calorie count. (And I won’t even go into how arbitrary calorie burn numbers are…) For example, I’m wearing both the Fitbit One and Fitbit Flex right now. It’s early in the morning, and I haven’t walked much yet (I work at home). The Flex shows that I’ve taken 582 steps; the One has counted 367 steps. That’s a huge difference. (A few minutes later, I haven’t taken any steps, and the Flex counted another six of them; because of my arm movements.)

It’s not just the inaccuracy of some of these devices that bothers me; it’s the fact that they are even on the market. If a device that claims to count your steps is not accurate, then it’s not performing it’s most basic task correctly. I understand that this is a difficult technology to perfect. The Fitbit One, worn in the correct location, is the most accurate; as for others, perhaps you simply can’t make an accurate wrist-worn tracker. Many people will buy these because they are very visible gadgets; but they’re little more than fashion accessories. If the basic information they give you is flawed, then there’s not much point.

Note that I could use my iPhone 5s to count steps, as it has a special chip that counts and records such information from its accelerometer. But my phone isn’t always in my pocket when I’m walking – when I walk on my treadmill, I put it on a window ledge – so it wouldn’t count everything. And when I walk around the house during the day, I often leave it on my desk.

As I said earlier, these devices are motivators; they can help you be more active. If you simply want an idea of your activity relative to other days, pretty much any device will give you that. If you want more accuracy, I’d recommend the Fitbit One, which counts your actual steps.

(I’d be interested in comments from readers who have any of these devices, or any others, as to their accuracy.)