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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 ๐œ‡.

So the 'standard error of the mean' is ๐‘†๐ท($\bar X$)=๐œŽ/๐‘›โˆš. If ๐œŽ is unknown, it is estimated as the sample standard deviation ๐‘†. This means that the '(estimated) standard error' is ๐‘†/$\sqrt 2$."

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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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