TABLE 1.

List of well-known SQIs available in the vital_sqi Python package.

SQI name	Type	Signal	Equation or brief description	Per segment	Per beat	Introduced as SQI in
Perfusion	Stats	PPG	$P_{\mathit{SQI}}=\frac{(y_{\max }-y_{\min })}{|\overline{x}|}\times 100$	Y	N
Kurtosis	Stats	PPG	$K_{\mathit{SQI}}=\frac{1}{N}{\sum }_{i=1}^N{[\frac{x_i-\overline{{\mu }_x}}{\sigma }]}^4$	Y	Y	Selvaraj et al. (2011)
Skewness	Stats	PPG	$S_{\mathit{SQI}}=\frac{1}{N}{\sum }_{i=1}^N{[\frac{x_i-\overline{{\mu }_x}}{\sigma }]}^3$	Y	Y	Krishnan et al. (2010)
Entropy	Stats	PPG	$E_{\mathit{SQI}}=-{\sum }_{i=1}^{N}x{(n)}^{2}lo{g}_e(x{(n)}^2)$	Y	Y	Selvaraj et al. (2011)
SNR	Stats	PPG and ECG	$N_{\mathit{SQI}}=\frac{{\sigma }_{\mathit{signal}}^2}{{\sigma }_{\mathit{noise}}^2}$	Y	N	Elgendi (2016)
Relative power	Stats	PPG and ECG	$R_{\mathit{SQI}}=\frac{{\sum }_{f=1}^{2.25}PSD}{{\sum }_{f=0}^{8}PSD}$	Y	N	Elgendi (2016)
Mean crossing	Stats	PPG	Number of mean crossings within the signal segment	Y	N
Zero crossing	Stats	PPG and ECG	Number of zero crossings within the signal segment	Y	N	Elgendi (2016)
MSQ	Morph	PPG	Degree of agreement between two distinct peak detector algorithms	Y	N	Elgendi (2016)
Correlogram	Stats	PPG and ECG	Location and prominence of peaks of the signal segment correlogram	Y	N	Pradhan et al. (2017)
Dynamic time warping	Morph	PPG and ECG	Template matching of a single period with a mathematically described ideal period	N	Y	Li and Clifford (2012)