confusion on outliers
It completely depends on the context of the data that is being considered. For example, $2\sigma$ from the mean ($\mu$), depends on the distribution of the data. What is the value of $\mathbb{P}(-2\sigma &lt; X - \mu &lt;2\sigma)$. 

Also, there are many methods for outlier detection, and all of them depend on the context. Hence, you cannot say which method should be used by taking it outside the context. You should do some experiments by these methods over the data, and then base on the real outliers sample, decide which method is proper for the current data.