Fig. 3

Visualization of decisions made by the XGBoost algorithm. A Top 20 most influential variables used by XGBoost. Interpretation: Each dot represents a variable for an individual patient instance. Variables pictured to the right side of the y-axis influenced the model to predict delirium, whereas variables to the left of the y-axis influenced the model against prediction of delirium. Red signifies a higher absolute value (numeric variables) or yes/present (categorical variables), and blue signifies a lower absolute value (numeric variables) or no/absent (categorical variables). For example: Higher age (red color) influenced the model to predict delirium (right of y-axis), whereas lower age (blue color) influenced the model toward prediction of no delirium. Decision path for a true negative (B) and a true positive (C) delirium prediction by XGBoost for two individual patients. Interpretation: The algorithm begins at the center of the x-axis with a baseline value. The model considers each variable along the y-axis one at a time (values shown in parenthesis), to influence the model toward making a positive (vertical line moves toward the right) or negative (vertical line moves toward the left) delirium risk prediction. For example: In panel B, variables which significantly influenced the model toward a negative delirium prediction include outpatient surgery, ASA class 1, low fall risk, not neurosurgery, and short case length. In panel C, variables which significantly influenced the model toward a positive delirium prediction include not oriented to place, older age, ASA class 4, unable to rate pain using numeric assessment scale, high pressure ulcer risk, unable to spell ‘WORLD’ backwards, and high fall risk. Abbreviations: ASA, American Society of Anesthesiologists; ICD-10, International Classification of Diseases, 10th revision; ICD-10 F00-F99, mental and behavioral disorders; kg, kilograms; ICD-10 Z00-Z99, factors influencing health status and contact with health services; ICD-10 G00-G99, diseases of the nervous system; ERAS, enhanced recovery after surgery; ICD-10 C00-D48, neoplasms; ICD-10 I00-I99, diseases of the circulatory system; ICD-10 N00-N99, disease of the genitourinary system