Table 2. The results of an assessment for bias in accordance with Hayden's criteria.

	Assessment domain
Study	study participation	study attrition	prognostic factor measurement	outcome measurement	confounding measurement and account	analysis	total score
Braun, 2006	moderate	low	moderate	low	low	low	10
Hyrich, 2006	low	low	unsure	low	moderate	low	9*
Bobbio, 2007	low	low	low	low	high	low	10
Lequerre, 2007	low	low	moderate	low	moderate	low	10
Mancarella, 2007	low	high	moderate	low	high	low	7
Wijbrandts, 2007	low	moderate	low	low	moderate	low	10
Cuchacovich, 2008	moderate	moderate	low	low	moderate	low	10
Wouter, 2008	low	low	low	low	moderate	low	11
Alexandra, 2009	moderate	low	moderate	low	moderate	low	9
Keystone, 2009	high	low	moderate	low	low	low	9
Potter, 2009	low	moderate	low	low	moderate	low	10
Soto, 2010	moderate	low	moderate	low	moderate	low	9
Vasilopoulos, 2011	low	low	low	low	moderate	low	11
Canhao, 2012	low	moderate	moderate	low	moderate	low	9

Unsure: not enough information to evaluate.

Low, low risk of bias; moderate, moderate risk of bias; high, high risk of bias.

*One of the domains was assessed as “unsure” due to unavailable information even after the authors were contacted.