Fig. 2. Comparison of propensity scores for the use of catecholamines by group and quartiles. The figure contains eight box plots. The  lower and  upper ends of each box denote the 25th and 75th percentiles, respectively. The  solid horizontal line through each shaded box denotes the median of the distribution, and the  cross denotes the mean. The  vertical solid black lines (whiskers) reach out to 1.5 × the interquartile range. The  circles above the whiskers denote individual extreme observations. The  bars represent the number of subjects included in each quartile.