Table 2.

Quality assessment of included studies. ○ = low risk of bias, ◐ = moderate risk of bias, • = high risk of bias.

Study	Study Participation	Study Attrition	Prognostic factor	Outcome	Study con-founding	Analysis and reporting		Total Risk of Bias
Balermpas 2013	○	•	◐	○	○	○	1	Low
Bron 2012	•	•	◐	◐	◐	◐	6	High
Distel 2009	◐	•	◐	○	◐	○	3	Moderate
Hasmim 2013	◐	•	◐	○	◐	◐	4	High
Kim 2016	◐	•	○	○	○	○	1	Low
Nasman 2012	○	•	◐	○	○	◐	2	Moderate
Nguyen 2016	○	◐	○	○	○	○	0	Low
Nordfors 2013	○	•	◐	○	○	○	1	Low
Oguejiofor 2015	◐	•	◐	◐	◐	○	4	High
Park 2013	◐	•	•	◐	○	○	4	High
Pretscher 2009	◐	•	○	○	•	○	3	Moderate
Punt 2016	◐	•	○	○	◐	○	2	Moderate
Van Kempen 2016	◐	•	◐	○	○	○	2	Moderate
Watanabe 2010	◐	•	◐	◐	◐	○	4	High
Wolf 2015	◐	•	○	○	○	•	3	Moderate
Zancope 2010	•	•	◐	○	○	•	5	High