Table 2:

Methodology quality scoring system

Question	Characteristic	Score
1. Representativeness	Not representative	0
	Representative	1
2. Number of hospitals or surgeons	Hospitals <20 and surgeons <50	0
	Hospitals ≥20 or surgeons ≥50	1
	Hospitals ≥20 and surgeons ≥50	2
3. Total sample size (cases)	<1000	0
	≥1000	1
4. Number of adverse events	<20	0
	21–100	1
	>100	2
5. Unit of analysis	Hospital or surgeon	0
	Both separately	1
	Both together	2
	Both together + further component	3
6. Appropriateness of patient selection	Not measured	0
	Measured separately	1
	Measured and analysed	2
7. Volume	Two categories	0
	Multiple categories	1
8. Risk adjustment	None	0
	Administrative data only	1
	Clinical data only clinical + 0 1 2 3	2
	Clinical data + C >0.75 and Hosmer–Lemeshow test positive	3
9. Clinical processes of care	Not measured 1 2	0 1 2
	≥2	2
10. Outcomes	Single outcome measured	0
	≥2 outcomes measured	1