Table 1 Inclusion and exclusion criteria for studies of the volume-outcome relationship in surgery

Inclusion criterion: studies of the hospital volume-outcome relationship in surgery, the methodology of which was described in sufficient detail for the extraction of all requisite data.

Exclusion criterion: studies of surgeon-specific volume-outcome relationships.

Inclusion criterion: methods used to assess the hospital volume-outcome relationship in surgery.

Exclusion criterion: studies in which (i) the hospital volume was a covariate in a model and (ii) the impact of volume per se was not apparent.

Inclusion criteria: all types of surgery, all types of patient-related outcome (length of stay, mortality, morbidity, cost, etc.), all countries.

Exclusion criteria: none.

Inclusion criterion: primary quantitative research publications written in English and published after 2009.

Exclusion criterion: studies with other designs.