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Using a bar over a variable seems to be standard shorthand notation for the mean value:

$\bar x = \frac{1}{N}\sum_i x_i$

Now, what is suitable shortand notation for the variance? I can only find $\sigma^2$ or $\mathrm{Var}$, but no diacritic (modifier) symbol that would make the notation more concise. Like this ring symbol, for example:

$\mathring x = \frac{1}{N}\sum_i (x_i - \bar x)^2$

What is a suitable symbol to denote the variance in such a compact manner?

Edgar
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1 Answers1

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Personnally, the thing I saw the most is $\mathbb{E}(X)$ for the mean value and $var(X)$ or $\mathbb{E}((X-\mathbb{E}(X))^2)$ for the variance.

The use of symbols depends on the context (there are just context-wise habits, no conventions), you just have to precise what you are talking about. For instance, in topology, both the bar and the ring get a different meaning (respectively adherence and interior).

Vincent
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  • $\mathbb{E}(X)$ and $\bar x$ are not one and the same thing. When $\bar x$ is used it, it is most often in a statistical context, where the $x_i$ are samples. – Jean Marie Sep 02 '16 at 14:21