# What's the meaning of negative accuracy for measurements of physical quantities?

What's the meaning of negative accuracy for measurements of physical quantities? Can measured values of a physical quantity ever have a negative accuracy?

The Wikipedia article http://en.wikipedia.org/wiki/Accuracy_and_precision explains the accuracy as defined for interpreting observed values of a random variable which has certain probability distribution. I am not sure how much the interpretation is applicable to measurement of physical quantities as probability isn't necessarily a well-defined physical quantity.

• Depends on you exact definition of accuracy. Sometimes this $| x_\text{observed} - x_\text{actual}|$, and sometimes it is a logarithm of that. – Sasha Nov 28 '11 at 21:03
• As with Sasha the meanings of "accuracy" I'm familiar with are generally non-negative. Do you mean the "residual" or the "fractional residual"? – dmckee --- ex-moderator kitten Nov 29 '11 at 1:45
• The only place where "negative" is found in the link you provide is in "binary classification", yes/no tests, and even there accuracy as defined is only a positive number. You must be misunderstanding something. – anna v Nov 29 '11 at 4:58
• Maybe You are not familiar with the math. symbol for absolut value? – Georg Nov 30 '11 at 9:42
• Can accuracy as defined as reference.wolfram.com/mathematica/ref/Accuracy.html has a nontrivial use/meaning when it's negative? It says "... With uncertainty dx, Accuracy[x] is -Log[10, dx] ..." – Computist Nov 30 '11 at 21:17