Table 3.

Summary findings for the primary outcome.

No	Outcome Significance	Trials (Year)	Quality of the Evidence (Grade)
1	No significant correlation	Wieselmann-Penker et al. (2001) [20]	+ + − − low due to indirectness, imprecision
2	Significant correlation	Conti et al. (2014) [15]	+ + − − low due to indirectness, imprecision
3		Gu et al. (2015) [24]	+ + + − moderate due to indirectness
4		Sato et al. (2015) [18]	+ + − − low due to indirectness, imprecision
5		Criado et al. (2016) [14]	+ + − − low due to indirectness, imprecision
6		Watanabe et al. (2011) [19]	+ − − − very low due to indirectness, imprecision, inconsistency
7		Goto et al. (2015) [22]	+ − − − very low due to indirectness, imprecision, inconsistency
8		Jadidi et al. (2008) [23]	+ + − − low due to indirectness, imprecision
9		Raphael et al. (2013) [21]	+ + − − low due to indirectness, imprecision
10		Sumiya et al. (2014) [25]	+ − − −very low due to imprecision, indirections, inconsistency

Quality of evidence: ++++ high, +++− moderate, ++−−low, +−−− very low.