Table 3.

Different equations for estimation of frequency indices in previous studies.

Studies	Equation
Peng et al. (2021)	$\mathit{\text{Normalized}}\mathit{\text{sum}}\mathit{\text{of}}\mathit{\text{spectrum}}\mathit{\text{amplitudes}}=\frac{{{\displaystyle \sum }}_{\mathrm{f}={\mathrm{f}}_1}^{{\mathrm{f}}_2}\left|\mathrm{Z}\left(\mathrm{f}\right)\right|}{{{\displaystyle \sum }}_{\mathrm{f}=4}^{30}\left|\mathrm{Z}\left(\mathrm{f}\right)\right|}$
Benda et al. (2018) and Cao et al. (2014)	$\mathit{\text{Mean}}\mathit{\text{of}}\mathit{\text{spectrum}}\mathit{\text{amplitudes}}=\frac{{{\displaystyle \sum }}_{{\mathrm{f}}_1}^{{\mathrm{f}}_2}\left|\mathrm{Z}\left(\mathrm{f}\right)\right|}{\mathrm{N}}$
Zheng et al. (2020) and Jap et al. (2009)	$\mathit{\text{Sum}}\mathit{\text{of}}\mathit{\text{spectrum}}\mathit{\text{amplitudes}}={\displaystyle \sum _{\mathrm{f}={\mathrm{f}}_1}^{{\mathrm{f}}_2}}\left|\mathrm{Z}\left(\mathrm{f}\right)\right|$
Chen et al. (2013)	$\mathit{\text{Sum}}\mathit{\text{of}}\mathit{\text{squered}}\mathit{\text{of}}\mathit{\text{spectrum}}\mathit{\text{amplitudes}}={\displaystyle \sum _{\mathrm{f}={\mathrm{f}}_1}^{{\mathrm{f}}_2}}{\left|\mathrm{Z}\left(\mathrm{f}\right)\right|}^2$