**Standard error** - **Wikipedia**
https://en.wikipedia.org/wiki/Standard_error_(statistics)

The formula may be derived from the variance of a sum of independent random variables.. If ,, …, are independent observations from a population that has a mean and **standard** deviation , then the variance of the total = (+ + ⋯ +) is .; The variance of / (the mean ¯) must be () =.; And the **standard** deviation of / must be /; Student approximation when σ value is unknown

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