Exposure variables
Primary exposure variable: Day-case or overnight stay. Day-case
was defined as any procedure where the admission day and discharge day
were the same.
Secondary exposure variable: In secondary analysis the exposure
variable was defined at a trust level; trusts operating on ≥ 80% of
patients as day-case and trusts operating on < 50% of
patients as day-case were compared. Eighty percent was chosen to reflect
trusts where day-case surgery appeared to be the default and 50% was
chosen to reflect trusts where surgery with overnight stay was dominant.
This analysis was conducted to minimise bias, when considering the data
at a patient level due to patients with more severe illness (and so
potentially poorer outcomes) being more likely to stay in overnight. For
this analysis, trusts performing fewer than 50 procedures over the study
period (average of ten per financial year) were excluded.