I am very confused as to what the correct way is to calculate the uncertainty of the average of values ($x_{avg}$) in a data set of measurements $(x_1 ... x_N)$. I have found at least four different ways of doing it around the internet, as follows:
Method 1: Uncertainty is the average of the deviations from the mean. That is, $$\Delta x_{avg} = \frac{(|x_{avg} - x_1| + ... + |x_{avg} - x_N|)}{N}$$ (as described in this Youtube video)
Method 2: $\Delta x_{avg} = \frac{R}{2}$, where $R$ is the range of the values (from this Youtube video)
Method 3: $\Delta x_{avg} = \frac{R}{2\sqrt{N}}$ from this document
Method 4: $\Delta x_{avg} = \frac{\sigma}{\sqrt{N}}$, $\sigma$ being the standard deviation of the data set (from here)
Which is the correct way?