Originally Posted by Kapusta

You are confusing precision with accuracy. No, I wasn't. Repeating the weight test, and getting the same value, shows it measures x weight accurately to weigh x. I'm not saying anything about how much I actually weigh, just that the scale reads my weight unchanged, and the total combined weight also the same. Therefore, subtracting one from the other should be relatively accurate, in actual pounds,(As I know I'm around 160, and the bike is sub-20.) More likely to be accurate than weighing either separately.

Plenty of bathroom scales read out to the first decimal place, but do not measure with accuracy of that level.

Also, if there is an error of 1lb in your weight and an error of 1lb in the you+bike weight, then the error of the.bike weight could be anywhere from 0 to two lbs. . You are making an assumption that any inaccuracies in measurement fluctuate randomly, rather than a particular type of error (either absolute, or on a percentage basis). I took multiple readings, that did not vary thus, if there is an error in absolute terms,it is a consistent error.

I have no way of knowing whether a reading of 158.2 is accurate or 177.2 is accurate in absolute terms. However, they are likely to be closely correlated. knowing that in absolute terms, the scale is within a few pounds.

Try this sometime; weigh yourself with an empty water bottle, then do so again with varying amounts of water in the water bottle and see how accurately the scale shows fhe difference. Here you may have a point, though again, I am less concerned with individual gradations of 0.1 lbs. I will weigh myself and myself with 2 gallons of water, Results tonight if batteries are still working
I’m sure some scales are good enough to do this. But most typical affordable home digital scales are not. At least the few that I have used.