[Pet_Ant]

How do ±6.4 and ±6.9 combine to ±9.4 and not ±13.3 ?

[fulvioterzapi]

Errors do not sum like regular numbers. You want to take the square root of the sum of the squares of the errors.

sqrt(6.4^2 + 6.9^2) ≈ 9.4

You can have a look here: http://ipl.physics.harvard.edu/wp-uploads/2013/03/PS3_Error_...

[stocknoob]

That PDF is great, thanks for sharing.

[alephxyz]

The variances are additive but not the std dev. Sqrt(6.4^2 + 6.9^2) = 9.4

[pif]

https://physics.nist.gov/cuu/Uncertainty/combination.html