Table 2. Quality assessment of studies enrolled using the Newcastle–Ottawa quality assessment scale.

Study [author (year)]	Selection				Comparability	Outcome			Scores
	1	2	3	4		1	2	3
Shangnao Xie (2014) [18]	★	★	★	–	★★	–	★	★	7
Faruk Tas (2014) [19]	★	★	★	–	★	★	★	★	7
B. K. Linderholm (2013) [20]	–-	★	★	★	★★	★	★	★	8
Natalia Krawczyk (2014) [21]	★	★	★	–	★★	–	–	★	6
Gerhard Schaller (1996) [22]	★	★	★	★	★	★	–	★	8
Ute Woelfle (2004) [23]	–	★	★	★	★★	★	★	★	8
Soo Kyung Ahn (2013) [24]	★	★	★	★	★★	★	★	★	9
Masahiro Takada (2004) [25]	★	★	★	–	★★	★	★	★	8
Maria Hagg Olofssion (2007) [26]	–	★	★	★	★★	–	★	★	7

★ Study basically meets the criteria; ★★ Study meets the criteria strongly.