Table 4.

Data extraction form

Title	___________________________________
Author	___________________________________
Year of publication	___________________________________
Research design	___________________________________
Research objectives	___________________________________
Type of sources	❒ Published ❒ Grey literature
Specific information	___________________________________
1. Database used	___________________________________
	❒ Electronic health record ❒ Administrative data ❒ Claims data ❒ Patient or disease register ❒ Health survey ❒ Clinical trials data
2. Surgery studied	___________________________________
❒ Head and Neck ❒ Oral ❒ Orthopaedics ❒ Paediatrics ❒ Plastics ❒ Vascular ❒ Thoracic and cardiovascular ❒ Visceral and digestive ❒ Neurosurgery ❒ ENT ❒ Obstetrics and gynaecology ❒ Ophthalmology ❒ Urology
3. Use of the International Classification of Disease	❒ Yes ❒ No
4. Inclusion and exclusion criteria	___________________________________
5. Method to sort hospitals by volume	❒ Continuous ❒ Categorical ❒ Both
If continuous or both, precise the method used:	❒ Fractals ❒ Statistical cut-off ❒ Arbitrary ❒ Other
6. Outcomes	❒ Mortality ❒ Cost ❒ Length of stay ❒ Readmission ❒ Other
If other, precise the interesting outcome studied	___________________________________
7. Severity score	❒ Charlson/Deyo ❒ Elixhauser ❒ Other
and covariates	___________________________________
8. Statistical methods	❒ One sample test ❒ Two independent sample test ❒ Two correlated sample test ❒ More than two independent sample test ❒ More than two correlated sample test ❒ Correlation ❒ Two categorical variables ❒ Same categorical outcome on matched pairs ❒ Linear multiple regression ❒ Other
If other, precise	___________________________________
9. Results’ presentation	❒ Graph(s) ❒ Table(s) ❒ Both
If graph or both, its precise typology	❒ Line graph ❒ Bar graph ❒ Scatter gram ❒ Histogram ❒ Pie chart ❒ Boxplot ❒ Other
10. Conclusion: positive volume-outcome relationship	❒ Yes ❒ No