Statistics basic: stddev and z-score

I’ve been trying to wrap my head around some statistics/data science used for dissecting ddos attacks, and came across a couple of new topics that are quite important but rarely explained.


Standard deviation

Standard deviation is a property of a set that describes the spread around the mean.

Sx = σ = de standard deviation of the set
Xi = The number i in the set.
Xgem = the mean of the set
Nx = the total number of elements in the set

σ = Sx = √( ∑ ( (xi – xgem)2 / nx) )



z-score: easy normalized way of seeing if something is above the average or below, and if it is an outlier (z-score >3 | <3 is often seen as a outlier)

 Z = frac{X - mu}{sigma}.
SRC: statistiekbegleider

mean = average
Z-score = (Measurement – mean) / stddev

In python:

df['zscore'] = ((df['count'] - df['count'].mean()) / df['count'].std(ddof=0)).round().fillna(NONE)

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