The parent is taking the error numbers as something like 1 standard deviation on a normal distribution, rather than an absolute limit. so, x = 6±3.5m means som

The parent is taking the error numbers as something like 1 standard deviation on a normal distribution, rather than an absolute limit. so, x = 6±3.5m means something like "x is equal to 6 + a random number taken from a normal distribution with standard deviation 3.5 centered around 0" and y is the same, but with a _different_ random number. (I'm a little hazy on exactly what distribution is being assumed, but the basic idea of x and y being different random numbers is the important thing) In principle one could also interpret x = 6±3.5m as "x is equal to 6 + a random number taken uniformly from the rand -3.5 to 3.5" or something, and then x+y would be 12+-7, but the real world numbers this is nominally supposed to represent don't really work that way. I think this is what is mean by "The best you probably can do is to forget about the second issue, and do interval arithmetic. That is useful, but gives you extremely pessimistic results."