Fig. 2

Histograms of standardized differences before and after propensity score-matching analysis. a A histogram of standardized differences before the propensity score-matching (PSM) analysis when the number of pleural surfaces crossed (NOPSC) was 1. b A histogram of standardized differences after the PSM analysis when NOPSC was 1. The standardized differences after matching are concentrated around 0, indicating good matching performance. c A histogram of standardized differences before the PSM analysis when NOPSC was 2. d A histogram of standardized differences after the PSM analysis when NOPSC was 2. The standardized differences after matching are concentrated around 0, indicating good matching performance. e A histogram of standardized differences before the PSM analysis when NOPSC was 3. f A histogram of standardized differences after the PSM analysis when NOPSC was 3. The standardized differences after matching are concentrated around 0, indicating good matching performance