Activity trackers are "all the same"

There has been a veritable explosion in the number and variety of activity trackers. They come in all shapes, sizes, colors, prices, and ancillary features. However the core functionality of an activity tracker is to count steps, and in this respect, one can argue that they are "all the same."

The centerpiece of an activity tracker is the accelerometer which can be used to monitor one's movement: "The accelerometer measures changes in velocity as you move. When you step forward, you accelerate, and when you plant your foot down after striding forward, you decelerate. This cyclical pattern of acceleration and deceleration can be converted into electrical signals that are counted." A key question is whether the different trackers vary in their step counting. This question has been addressed in several product reviews.

First up was was a survey from The Verge which compared 8 activity trackers. The author found the following:

"All the services clocked roughly the same amount of steps for the run. The S4 counted the least number of steps at 2,526, followed by the 2,535 steps counted by the Jawbone Up. The Pulse and the Fitbit Zip were nearly identical, with 2,539 and 2,540 counted steps, with the Fitbit Flex somehow measuring five more at 2,545. The run earned me 590 Nike+ Fuel points, which is about a fifth of the recommended activity for an active day."

A more systematic study was performed by a reviewer for the technology website Wirecutter who tested 4 trackers over a 6 day period and plotted the steps per day (Figure 1):

Figure 1. Steps per day over 6 days for the Fitbit One, Withings Pulse, Fitbit Force, and Jawbone UP24 (plot copied from Jim McDannald at Wirecutter). 

As you can see, the number of steps are within 5-10% for all the trackers. The Fitbit Force counted the most steps and the Jawbone UP24 the least, but they are pretty similar.

Finally, Consumer Reports performed perhaps the most definitive study that "combined step count accuracy for walking for five minutes on a level treadmill at 3 mph, using an elliptical exerciser for five minutes, walking up and down several flights of stairs and picking up toys from the floor." They tested 6 trackers and gave all of them either an excellent or good score for step counting accuracy.

In summary for the core functionality of tracking steps, the majority of activity trackers perform in an almost identical fashion. These relatively minor differences are likely to be small compared to other uncertainties when estimating the number of calories consumed such as variations in 1) untracked activities (e.g. moving your legs while keeping your arms and body still), 2) the intensity or type of step (long or short), and 3) individual differences in calorie burn rate for a given athletic exertion (i.e. different metabolic rates for every person).

There is greater product differentiation in the ancillary features among the activity trackers:

1. User interface (UI), some have a screen while others do not 
2. Communication with computer or mobile device by wireless (Bluetooth) or cable (USB) 
3. Software and web services to store, analyze and display activity data 
4. Form factor (clipped, wristband, armband, headband, smartphone, etc.)
5. Additional capabilities such as sleep tracking or logging calorie intake
6. Appearance/style
7. Price

The last variable is important to most people, and there is a vast difference in price range from free apps on your smartphone to $200+ for more sophisticated devices; the typical activity tracker costs around $100. 

One exciting trend are plans for trackers to include more medical functionality. For example future devices can be expected to measure variables like your heart rate, blood pressure, and even blood glucose levels. In this manner, valuable health information can be collected along with your activity data.