Inclusion and exclusion criteria for index procedure
We identified ESS using the Office of Populations Censuses and Surveys Classification of Interventions and Procedures version 4 (OPCS-4) codes: E081, E132, E133, E142, E143 or E148 were used in the first position in the procedural record together with the code Y761 (for functional ESS) anywhere in the procedural record. Descriptions of the definitions of these codes are given in Supplementary material Table S1.Patients were only included if the clinical specialty listed was ENT.
Patients were also excluded where:
  1. The procedure was non-elective
  2. The patient was aged <17 years.
  3. Patients with a diagnosis of any of the following conditions anywhere in the diagnostic record for the index admission: malignant neoplasm, vasculitis, granulomatosis intracranial and intraspinal abscess and granuloma, granuloma and granuloma-like lesions of oral mucosa, cystic fibrosis, diseases of bronchus and cerebrospinal fluid leak without diagnosis of sinusitis or nasal polyp. Patients undergoing trans-sphenoidal pituitary surgery were also excluded. The OPCS-4 and International Statistical Classification of Diseases and Related Health Problems, tenth revision (ICD-10) codes used to define these procedures are given in Supplementary material Table S1 .
Patients with malignant neoplasm and with the other conditions described would not routinely be considered for day-case surgery, although some are likely to be suitable for, and undergo day-case surgery.
To ensure all datapoints were independent of one another at a patient level, only the chronologically first recorded index procedure for anyone who had more than one procedure during the study period was included in the dataset. The data extraction process is summarised inSupplementary material Figure S1 .