The effect of date of surgery on the odds of mortality. This figure illustrates the effect of date on the log odds of postoperative mortality rate, relative to the earliest time period point (January 2014). There does not appear to be much effect until roughly March of 2015 (with exception of 48 h), where there is a sharp decline in postoperative mortality rate, adjusting for the effects of all other factors in the multiple regression. The postoperative mortality odds ratio (OR) associated with day 450 versus day 0 (January 1, 2014) is 0.55 (95% CI, 0.25–1.23; P = 0.15), 0.43 (95% CI, 0.24–0.76; P = 0.003), and 0.58 (95% CI, 0.38–0.88; P = 0.01), for 24-h, 48-h, and 7-day mortality, respectively. The corresponding OR for day 600 versus day 450 is 0.11 (95% CI, 0.02–0.71; P = 0.02), 0.09 (95% CI, 0.03–0.33; P < 0.001), and 0.13 (95% CI, 0.05–0.35; P < 0.001), for 24-h, 48-h, and 7-day mortality, respectively.