[rodionos]

Other than normal distribution assumption, there is another assumption that doesn't hold true for most time series related to human activity, nature, or scheduling. If heteroscedasticity is present, you cannot use the same standard deviation for the entire series. A more practical approach is to compute variance for each calendar period separately.

Here's an example - expected variance for the number of SWIFT payments processed during non-banking hours is 0. Transaction counter greater than 0 is an outlier.