Table 3.

Statistical characteristic parameters of time domain and frequency domain.

Time–Domain Feature Parameters		Frequency–Domain Feature Parameters
Feature	Equation	Feature	Equation
Mean value	$T_1=\left(1/n\right){\displaystyle {\sum }_{i=1}^{n}x\left(i\right)}$	Mean value	$F_1=\left(1/l\right){\displaystyle {\sum }_{k=1}^{l}sp\left(k\right)}$
Standard deviation	$T_2=\sqrt{\left(1/(n-1)\right){\displaystyle {\sum }_{i=1}^n{\left(x\left(i\right)-T_1\right)}^2}}$	Standard deviation	$F_2=\sqrt{\left(1/(l-1)\right){\displaystyle {\sum }_{k=1}^l{\left(sp\left(k\right)-F_1\right)}^2}}$
Skewness	$T_3={\displaystyle {\sum }_{i=1}^n{\left(x\left(i\right)-T_1\right)}^3}/\left((n-1)T_2^3\right)$	Skewness	$F_3={\displaystyle {\sum }_{k=1}^l{\left(sp\left(k\right)-F_1\right)}^3}/\left((l-1)F_2^3\right)$
Kurtosis	$T_4={\displaystyle {\sum }_{i=1}^n{\left(x\left(i\right)-T_1\right)}^3}/\left((n-1)T_2^3\right)$	Crest factor	$F_4=\max \left|sp\left(k\right)\right|/\sqrt{\left(1/l\right){\displaystyle {\sum }_{k=1}^{l}x{\left(k\right)}^2}}$
Crest factor	$T_5=\max \left|x\left(i\right)\right|/\sqrt{\left(1/n\right){\displaystyle {\sum }_{i=1}^{n}x{\left(i\right)}^2}}$	Shannon entropy	F5 = Equation (16)
Here x(i) is time series of an IMF for i = 1,2,…,n, n is the number of data points.		Here sp(k) is the envelope spectrum of an IMF for k = 1,2,…,l, l is the number of spectrum components.