In statistics, is an uppercase x-bar used as a notation for something?

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I was reading someone's answer and I came across him using the uppercase x-bar as a notation:

"Technically, the standard error is the standard deviation of an estimator. Most commonly, this refers to sample mean $\bar X$ as an estimator of the population mean 𝜇.

So the 'standard error of the mean' is 𝑆𝐷($\bar X$)=𝜎/𝑛√. If 𝜎 is unknown, it is estimated as the sample standard deviation 𝑆. This means that the '(estimated) standard error' is 𝑆/$\sqrt 2$."

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1 Answer

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If your sample is $X_1, \ldots, X_n$, then $\overline{X} := \frac{1}{n} \sum_{i=1}^n X_i$. Sometimes lowercase $x$ is used in place of $X$. See the Wikipedia page for more detail.

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