Figure 5.

Relationship between reported correlation coefficients (r) of estimated vs. reference SBP and DBP and sample size (n) across all biosignal modalities. M1: modality 1 (PPG + ECG), M2: modality 2 (PPG + BCG), M3: modality 3 (PPG + SCG), M4: modality 4 (PPG + IPG), M7: modality 7 (PPG + ECG + ICG), M8: modality 8 (PPG + ECG + BCG), M10: modality 10 (PPG + ECG + SCG). Study strength (represented by the dashed line): the positive relationship between r and n in each study. If a study is able to achieve a high correlation over a large group of participants, it is less likely to be due to chance and it can be concluded that the correlation values are more likely to be true and validated over a variety of individuals. Therefore, data points are plotted by the strength of the correlation to the sample size and therefore, data points in the top right corner are considered to be the strongest, whereas those in the bottom left corner are considered to be the weakest. M1 is the modality with the most data points and the strongest r. M1 is also the only category to include studies with sample sizes over 30, making the correlations stronger than those of studies with small sample sizes. Across most modalities, SBP estimations have consistently higher correlations with real SBP than DBP estimations do with real DBP [17,21,23,25,27,40,43,44,51,52,55,60,61,65,81,82]. Three studies from M2 [15], M1 [59], and M1 [67] showed the opposite trend and two studies from M1 [31] and M4 [20] reported the same r for both SBP and DBP. Note that M5 (modality 5: PPG + SBS), M6 (modality 6: PPG + ICG), and M9 (modality 9: PPG + SCG + GCG) are not included in the above figure as these studies did not report correlation values for SBP and DBP.