Subgroup Definitions

Given the large sample size, we conducted subgroup analyses to assess for interaction between patients (sex, age, BMI, comorbidity, hospital characteristics, and demographics of patients) and to assess the association between daylight levels and outcomes. The subgroups consist of 10 main groups as defined in Table 1: (1) Sex, (2) Age, (3) BMI, (4) LOS, (5) Surgery, (6) Diagnosis, (7) Comorbidity, (8) Patients' Demographics, (9) District, and (10) Education level. Age was categorized into four groups according to the median and interquartile range values in all participants (<51, 51–59, 60–67, and ≥68 years). Body-mass index (BMI) was calculated as weight in kilograms divided by the square of the height in meters (kg/m²) and was categorized into four groups according to cut-off values indicated in The BMI criteria adopted by Chinese Adults Overweight and Obesity Prevention and Control Guidelines: underweight (<18.5 kg/m²), normal range (18.5–24 kg/m²), overweight (24–30 kg/m²), and obese (≥30 kg/m²) (15). LOS was categorized into four groups: 1–6, 7–13, 14–29, and ≥30 days). The diagnosis was categorized into six groups: benign tumor, malignant tumor, inflammation, hernia, intestinal obstruction, and others. The district was divided into two groups: rural area and urban area. Education level was categorized into five groups: illiterate (defined as never received formal education), primary school, middle school, high school, and university degree. To find out the differences between the groups, we further divided subgroups into plural subgroups. A total of 616 plural subgroups were generated for further comparisons.

Basic characteristics of patients on low light and high light sides of the hospital unit.

^aVariable names are presented as categorical label [name].

^bVariables are listed as columnwise no. (percentage).

^cp-value is determined by a chi-squared test between low and high light sides.

P-value less than 0.05 will be in bold.