Error bar

They give a general idea of how precise a measurement is, or conversely, how far from the reported value the true (error free) value might be.

These quantities are not the same and so the measure selected should be stated explicitly in the graph or supporting text.

Error bars can be used to compare visually two quantities if various other conditions hold.

It has also been shown that error bars can be used as a direct manipulation interface for controlling probabilistic algorithms for approximate computation.

[2] A notorious misconception in elementary statistics is that error bars show whether a statistically significant difference exists, by checking simply for whether the error bars overlap; this is not the case.