A total of 155 362 patients were included in the analysis. Adjusted rate ratios and cost ratios were estimated using multivariable generalised linear mixed models with a log link to compare economic outcomes between pre-implementation and implementation periods in intervention and control hospitals (see models specification in online supplemental appendix). A negative binomial distribution was used to model the mean number of hospital bed-days per patient within 30 days following surgery, and a gamma distribution to model mean hospital costs reimbursed for this care per patient by the insurer. Adjusted RRR and RCR captured the control chart impact by comparing the change in outcomes from the pre-implementation with implementation periods between the intervention and control hospitals based on a difference-in-difference approach. A RRR or RCR value less than unity indicated improvement caused by control charts in intervention versus control hospitals. Estimates with corresponding 95% CI considered clustering of patients at the hospital level. Outcomes were adjusted for the patient expected number of healthcare consumption, death status within 30 days following surgical procedure and their interaction. The patient expected number of healthcare consumption was introduced in models as a categorical variable (quartiles), and considered age, sex, presence of comorbidities, emergency admission, date and operative procedure, main diagnosis, surgical procedure complexity, median household income for patient-level covariates and status for hospital-level covariates. €1.00 (£0.83; US$1.09).

RCR, ratios of cost ratios; RRR, ratios of rate ratios.