Table 4.

Quality assessment of included studies according to the QUIPS tool [5]

Study (year)	Study participation	Study attrition	Prognostic factor measurement	Outcome measure	Study confounding	Statistical analysis
Lu et al. [6] (2014)	Low	Low	Moderate	High	Low	Low
Luminari et al. [7] (2014)	High	Low	High	High	High	Low
Zinzani et al. [8] (2013)	Low	Low	High	High	Low	Low
Dupuis et al. [9] (2012)	Low	Low	Moderate	High	Low	Low
Trotman et al. [10] (2011)	Low	Low	High	High	Low	Low
Le Dortz et al. [11] (2010)	Low	Low	Low	High	Low	Low
Bishu et al. [12] (2007)	Low	Low	Moderate	High	Low	Low
Zinzani et al. [13] (2007)	Low	Low	High	High	Low	Low

Low low risk of bias, Moderate moderate risk of bias, High high risk of bias