Table 2

Study quality domains by study design, mean (SD)

Selection
(maximum 4)		Comparability
(maximum 2)		Outcome
(maximum 2)		Total
(maximum 8)
Study design
 Cross-sectional2.1 (1.0)0.9 (1.0)0.9 (0.6)3.9 (1.6)
 Ecological0.4 (1.0)0.0 (0.0)0.6 (0.5)1.1 (1.2)
 Cohort2.7 (0.8)0.6 (0.9)1.2 (0.6)4.4 (1.2)
 Case–control2.3 (0.6)1.3 (1.2)2.0 (0.0)* 6.7 (1.5)†
Year of publication
 1999 or earlier1.2 (1.3)0.2 (0.7)0.8 (0.8)2.3 (2.6)
 2000–20091.5 (1.5)0.3 (0.7)0.9 (0.5)2.8 (1.9)
 2010 or later2.2 (0.9)1.1 (1.0)1.0 (0.6)4.3 (1.4)

  • ↵*Maximum 3.

  • †Maximum 9.