Table 3.

Items of the Newcastle–Ottawa Scale.

Item	Points
	Yes	No	Not known
Selection
1. Representativeness of the sample	⊕ representative (random)	⊗ not representative (selected)	⊘ do not know
2. Sample size	This item is not relevant here
3. Non-respondents	⊕ comparable + high response rate	⊗ not comparable + low response rate	⊘ do not know
4. Ascertainment of exposure	⊕ validated	⊗ not validated/no description	⊘ do not know
Comparability
5. Comparability of subjects	⊕ comparable/controlled	⊗ not comparable/not controlled	⊘ do not know
Outcome
6. Assessment of outcome	⊕⊕ independent/blind linkage ⊕ self-report	⊗ no description	⊘ do not know
7. Statistical test	This item is not relevant here

⊕ = +1 point; ⊗ = −1 point; Ø = 0 point.