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Table 1 Nonlinear Heart Rate Variability measures selected in the current study

From: Nonlinear Heart Rate Variability features for real-life stress detection. Case study: students under stress due to university examination

Measure Unit Description
SD1 ms The standard deviation of the PP perpendicular to the line of identity
SD2 ms The standard deviation of the PP along to the line of identity
En(0.2)   Approximate Entropy computed with the threshold r set to 0.2*SDNN
En(r max )   Approximate Entropy computed with the threshold r
set to value which maximizes entropy
En(r chon )   Approximate Entropy computed with the threshold r set
to value computed with the formula proposed by Chon[28]
D 2   Correlation Dimension
α 1   Short term fluctuation slope in Detrended Fluctuation Analysis
α 2   Long-term fluctuation slope in Detrended Fluctuation Analysis
l mean Beats Mean line length in RP
l max Beats Maximum line length in RP
REC % Recurrence rate
DET % Determinism
ShEn   Shannon Entropy