Table 3.

Risk-of-bias assessment of the included studies, according to the modified Newcastle-Ottawa Scale (NOS)

NOS items / Study ID	Wu 2017	Kevin 2017	Levent 2016	Yin 2014	Mahaluxmivala 2005
Is the case definition adequate?	★	★	★	★	★
Representativeness of the cases	★	●	★	●	★
Selection of controls	●	★	●	●	●
Definition of controls	★	★	★	★	★
Study controls for the most important factor (i.e., age)	★	★	●	★	●
Study controls for the second important factor (i.e., sex)	★	★	★	★	●
Was the measurement method described?	★	★	★	★	★
Were the methods of measurements similar for cases and controls?	★	★	★	★	★
Non-response rate	★	★	★	●	★
Total Score	8	8	7	6	6

★was awarded when the respective information was available

● was awarded if the respective information was unavailable