"""
cellphe.features.helpers
~~~~~~~~~~~~~~~~~~~~~~~~
Helper functions for use in both frame and time-series level feature
calculations.
"""
from __future__ import annotations
import numpy as np
[docs]
def skewness(x: np.array) -> float:
"""Calculates the skewness of a sample.
Uses the type 2method in the R e1071::skewness implementation, which is
the version used in SAS and SPSS according to the documentation.
:param x: Sample.
:return: A float representing the skewness.
"""
mu = x.mean()
n = x.size
deltas = x - mu
m2 = np.sum(np.power(deltas, 2)) / n
m3 = np.sum(np.power(deltas, 3)) / n
g1 = m3 / np.power(m2, 3 / 2)
return g1 * np.sqrt(n * (n - 1)) / (n - 2)