Fitbit becoming less accurate over time?

April 1, 2014 10:21 AM

I have a Fitbit flex I've been wearing for about eight months now. In the past couple months I've started tracking my runs with the RunKeeper app, and when I compare that with the fitbit the flex is way off --- up to 20% off in distance. The weird part is, this seems to be a new thing --- when i first got the flex I compared it a few times with google maps of known distances, and I wasn't getting these huge discrepancies. I tried manually entering stride length in the fitbit settings, but doesn't seem to have helped. Any suggestions as to why this might be happening? Has anyone else noticed this with their flex?

Other notes: it's a bit off and on, this inaccuracy, and seems to be way worse with running than walking. For instance, I have about a 12 min walk between my subway stop and my office; google maps has this distance as .7 miles, the fitbit recorded me this morning as walking .74 in that 15 min stretch where I walked to work.

Could it be that I somehow deeply fucked up my stride length measurements? When I first noticed the discrepancy with the running app and the fitbit, I went to their help forums, which suggested entering my stride length into the profile settings. I went to a local track, and went back and forth on a 100 meter stretch several times counting paces, first walking and then jogging at a moderate pace. I'm 5'4 and I calculated my walking stride length as 2'1.7" and my running stride as 3'1.9"

I know there's some way of putting the device into "activity mode" to record workouts, but as I've mostly been using it for running and walking I've never bothered to figure that out --- would that help at all?

I have been working on my running these past several months, and while I'm still pretty damn slow I'm certainly faster than I was when I first got the flex and was starting off with a run/walk program. Could it be that?

posted by Diablevert to Health & Fitness (9 answers total)

I would suggest that it could also be your Runkeeper app. I use MapMyWalk and walking the exact same walks never gives me the same distances each time. Sometimes I walk with a group and if two of us are using the same app, we can be up to .5 miles off of each other on a 6 or so mile walk. I think all of these apps and devices have a fairly high margin of error. I would start looking at all of them as estimates rather than exact results.
posted by cecic at 10:27 AM on April 1, 2014

Response by poster: I should have clarified -- I did compare the runkeeper numbers against google maps and they were pretty close (I run mostly on city streets).
posted by Diablevert at 10:35 AM on April 1, 2014

I use a Jawbone Up band and was curious about accuracy as well, so I used it on a treadmill and found it to be slightly off. The Jawbone app had an option to calibrate it -- basically, I ran a mile and then selected the activity length that was off and told it that it was a mile. That set things straight. Perhaps instead of trying to calibrate against a GPS based app, try to check it against something fixed like a 400m track or a treadmill. I would do that before turning to the GPS based apps (or even trying to use a route map) since they seem far more prone to errors beyond your control. For example, I have a few GPS hiking apps, and one lets you see have pretty fine-grained control over your tracks -- well, occasionally the signal bounces so a route along a street might jump from one side to the other over the course of just a couple readings -- over a long run or hike those little bounces could really add up.
posted by This_Will_Be_Good at 11:04 AM on April 1, 2014 [1 favorite]

Hi, fellow fitbit user! I love my flex and I'm also a runner.

"I have been working on my running these past several months, and while I'm still pretty damn slow I'm certainly faster than I was when I first got the flex and was starting off with a run/walk program. Could it be that?"

My first guess is that this is the problem — you have become a more efficient runner. The easiest way to troubleshoot this is to go back to that 100m track and re-measure your run stride (walk stride is unlikely to have changed much).

Also, do you ever put your fitbit into sleep mode? You can put it into activity mode the exact same way — more information here under No. 3.
posted by Brittanie at 11:23 AM on April 1, 2014 [3 favorites]

My guess is that your stride and pace have changed. Stride/pace measurements are going to be much less accurate than GPS because your stride and pace will be variable and there is no way for fitbit to tell. Previous accuracy may have been because your stride was a floor effect and now that you are more fit your stride may have shortened or cadence increased (which is what faster runners do).

Are your changes with fitbit all showing you traveling further? All error in one direction is a calibration problem.
posted by srboisvert at 12:11 PM on April 1, 2014 [1 favorite]

My partner's Flex and my One show very different distances/step counts/active minutes when we spend the whole day together (like these last few days exploring Prague on vacation). His stats are always substantially more than mine.
posted by avocet at 3:49 PM on April 1, 2014

Response by poster: I don't want to thread hog, but i thought I'd just add some more data, since I really appreciate the hive mind helping me tussle this out ---

1) I bought the flex 8 months ago, checked the accuracy a couple times at that time, and was fine with it. I didn't actually calculate my stride and attempt to recalibrate it until just a few weeks ago, after i noticed the inaccuracy problem. So I'm sure I'm faster than I was 8 months ago (when the accuracy seemed pretty good) but I don't think I'm any faster than i was three or four weeks ago when I measured my stride the first time. Still, i could have fucked up the measurement somehow.

2) I went for a run just now. I tried tapping the flex to put it into activity mode as suggested above. Here's what I'm seeing: The RunKeeper app shows the distance as 4.49 miles. This run mapping website, which uses google maps, shows my route as 4.4 miles. The flex, however, shows 4.99 miles --- exactly half a mile off, or an 11% difference. Weirdly, Flex and RunKepper have very similar estimates for my calories burned despite being so far apart in the distance.

Maybe more importantly, the flex is also showing me at 7 miles for the day, and I think that's way too high --- i walked about three quarters of a mile between the subway and my office, and I went across the street to get lunch, but other than that, trust me, I was on my duff most of the day.

Gah, this is just so frustrating. I don't feel like I'm doing anything differently, but I don't think I can trust the device anymore, and I'm not sure what to do about it. I really liked having it as a little bit of a prod to get me to be more active, but it takes a lot of the satisfaction away from hitting a goal if I'm not sure I'm actually hitting it.
posted by Diablevert at 4:28 PM on April 1, 2014

Have you contacted Fitbit? I have found their customer service to be incredibly top-knotch mind-blowingly accommodating.
posted by Brittanie at 7:11 PM on April 1, 2014 [3 favorites]

Response by poster: Update for any future people who come across this thread: I think I found the issue. Apparently, at some point I shoved my phone in my pocket with the fitbit screen on or something, and accidentally changed a bunch of info on my profile screen to gibberish --- I didn't actually realise what I'd done until I went to add a friend to my friend list and saw the changed profile name.

One of the bits of info that got changed was the height --- it had me down as about 3 feet tall. Once I changed that back to my actual height the inaccuracy problem seemed to go away -- it's not perfect, but it's now much closer to other data I've checked for my running routes and such.

Interestingly, one of the bits of profile data that had not been changed by my pocket dial were the running and walking stride lengths I had manually entered. The messed-up height data seemed to override this somehow.
posted by Diablevert at 12:32 PM on June 9, 2014