When you want to add or subtract average values
reported with their standard deviations, you don't simply add the standard deviations to produce the
final standarddeviation. Instead, you square the standard deviations, add them,
then take the square root of the sum.
- Stot = sqrt(S12 +
S22 + ...)
Note that you will add the squares of
the errors even if you are subtracting the actual values.
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