FitBit Margin of Error

keithcw_the_first
keithcw_the_first Posts: 382 Member
in Health and Weight Loss
I posted this in the FitBit usergroup but I thought it would be of general interest to people using activity trackers and questioning their sanity.

Okay, I've given the Charge HR a chance for about three months now. That's generated a decent amount of data. I've known pretty much since day one that it's over-estimating calorie burn but I hadn't known by how much.

The Method

I've been doing bodyfat measurements regularly and eating at what I think is a deficit. That's supported by my overall weight loss. I haven't magically gained pounds and pounds of muscle in three months, so the number and measurements suggest that the small amount I've lost has been fat. That gives me confidence that there aren't muscle gains offsetting the scale weight.

Based on that I say x number of pounds lost over y period of time (times 3500) gives me a total number of calories burned over that time period. I divided that by the number of days to get an apparent daily deficit.

Then I took FitBit's measured daily activity and subtracted my calories in. I've been logging as well as I can: weighing in grams, all that. By simply subtracting my intake from the reported burn, I received another higher deficit number.

Then I just compared the two; my derived deficit based on pounds lost vs. the calculated deficit from FitBit's data dump.

The Results

FitBit's numbers would have suggested an overall loss of 16 pounds over the time period. This was demonstrably false and as I said, without any increase in lean body mass I can't say "Oh well maybe I gained soooo much muscle.

Actual fat loss over the measured period was nine pounds.

Basically, my FitBit is overestimating my daily activity by about 10%. The math is ugly but I believe it's done correctly and basically to make 16 pounds 9 pounds, I have to knock 10% off of every day's reported burn.

TL:DR

One, this restores my confidence in my Charge HR a tiny bit. I always knew it couldn't be accurate but I wasn't sure by how much.

Two, this gives me a way to manipulate the food plan feature that FitBit offers. I know that if it tells me I've hit a 1,000 calorie deficit for the day, it's probably only 900 calories. If I set it to the moderate plan (750, which is where I have it set) and I nail the goal, I'm probably only running a deficit of 675 calories. And so forth.

I can't speak to everyone's devices; this is all dependent on my accuracy of food logging, plus the source of most of my active calories (chores and strength training).

Just know that yes it is overestimating, and yes you can figure out roughly how much.

Replies

  • aprilh576
    aprilh576 Posts: 50 Member
    I think most fitness tracker overestimate calories burn that's why people constantly say weight loss is 80% food 20% exercise. I use fitbit mainly for movitation and step counting to keep me active. If I go by calories burned I will be losing more than 1-2lbs a week. But this is good information to read. Thanks for sharing.
  • LovingLife_Erin
    LovingLife_Erin Posts: 328 Member
    Interesting. I have a flex, and I usually lose within .5lb of what I should, based on my calories in vs. what it says my calories out are. I think I'm just a lucky one that it seems to work out well for me on average. I'm sure there is a margin of error though as it is just an estimate at the end of the day.
  • StaciMarie1974
    StaciMarie1974 Posts: 4,138 Member
    I generally have recommended people to assume a 5-10% error with any tracker, unless & until you have reason to think otherwise. I do think the error margin will depend on the type of activity a person does. Though my thinking is based on general trackers, not heart rate based.

    I know my Fitbit is accurate for me. When I hit maintenance and continued to eat just below my Fitbit daily burn, I continued to lose weight. I need to eat at/above. But it will vary a little for everyone I imagine.
  • shadow2soul
    shadow2soul Posts: 7,692 Member
    This will vary from person to person. For me, my Surge averages out to be almost perfect (slightly under if we want to be technical).
  • onyxgirl17
    onyxgirl17 Posts: 1,722 Member
    I took a straight 250 off of it for my meal plan. I think it overestimates my activity burn and underestimates my BMR though.
  • keithcw_the_first
    keithcw_the_first Posts: 382 Member
    shadow2soul wrote: »
    This will vary from person to person. For me, my Surge averages out to be almost perfect (slightly under if we want to be technical).

    For sure I would imagine it varies. And if I started training for a 5k or something I bet the error margin would change, especially since FitBits seem to favor step-based activities.

    It was foolish of me to think that the addition of continuous HR monitoring would create a perfect machine.
  • kcjchang
    kcjchang Posts: 709 Member
    Part of the error could be due to the duration component in the algorithm used to calculate calories. It doesn't know if you walk 30 min and sit for 30 min verse walk for one hour; during the one hour period, it counted the same number of steps. The two examples will give a very different results in amount of calories burned especially if it assume the two events are cover at the same pace.

    See https://dl.dropboxusercontent.com/u/7644128/Step Counters.txt. It's was posted for Endomondo users, a MFP's sister site.

    It's a glorified pedometer and count steps really well.
  • brightsideofpink
    brightsideofpink Posts: 1,018 Member
    kcjchang wrote: »
    Part of the error could be due to the duration component in the algorithm used to calculate calories. It doesn't know if you walk 30 min and sit for 30 min verse walk for one hour; during the one hour period, it counted the same number of steps. The two examples will give a very different results in amount of calories burned especially if it assume the two events are cover at the same pace.

    See https://dl.dropboxusercontent.com/u/7644128/Step Counters.txt. It's was posted for Endomondo users, a MFP's sister site.

    It's a glorified pedometer and count steps really well.

    I have only tried a zip, but I imagine they work the same. My fitbit counts in 15 minute intervals, not one hour. So yes, it would know if I sat on my rear for 30 minutes.

    I have had mine for 9 months, and my projected loss with fitbit counting as my activity tracker has been spot on. I set to sedentary though so it doesn't give me a whole lot for default daily burn unless I earn it.
  • shadow2soul
    shadow2soul Posts: 7,692 Member
    brightsideofpink wrote: »
    kcjchang wrote: »
    Part of the error could be due to the duration component in the algorithm used to calculate calories. It doesn't know if you walk 30 min and sit for 30 min verse walk for one hour; during the one hour period, it counted the same number of steps. The two examples will give a very different results in amount of calories burned especially if it assume the two events are cover at the same pace.

    See https://dl.dropboxusercontent.com/u/7644128/Step Counters.txt. It's was posted for Endomondo users, a MFP's sister site.

    It's a glorified pedometer and count steps really well.

    I have only tried a zip, but I imagine they work the same. My fitbit counts in 15 minute intervals, not one hour. So yes, it would know if I sat on my rear for 30 minutes.

    I have had mine for 9 months, and my projected loss with fitbit counting as my activity tracker has been spot on. I set to sedentary though so it doesn't give me a whole lot for default daily burn unless I earn it.

    @brightsideofpink - the graph on the dashboard breaks it up into 15 minute segments, but the graph in the activity log breaks it up even further into 5 minute segments.

    I have no clue what that other person is talking about. Does endo have a step counter in the app maybe?

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