Study Year of
publication		 Sample
size		 Case
selection
quality		 Validity of
methodology		 Statistical
analysis
adequacy		 Confounding
variables
adequately
addressed		 Results
reporting		 Judged
quality
Ritz et al. [12] 2002 + - + + + + High
Chung et al. [17] 2012 - + - + - - Low
Dolk et al. [18] 2010 + + + + - + High
Gilboa et al. [19] 2005 + + + + + + High
Hansen et al. [20] 2009 - - + + - + Medium
Hwang et al. [21] 2008 + - + + + + High
Marshall et al. [22] 2010 + + + + - + High
Rankin et al. [23] 2009 - + + + + + High