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

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/$$\sqrt 2$$."

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.