Table 2. Assessment of Quality of Methodology of the Studies Using the Newcastle-Ottawa Quality Assessment Scale.

Author	1	2	3	4	5	6	7	8	9	Total score
Robman [15]	1	1	1	1	1	1	1	1	1	9
Ishida [11]	0	1	0	1	1	0	1	1	1	6
Kalayoglu [10]	1	1	0	1	0	0	1	1	1	6
Miller [14]	1	1	0	1	1	0	1	1	1	7
Klein [31]	1	1	1	1	1	1	1	1	1	9
Haas [32]	1	1	0	1	1	0	1	1	1	7
Shen D [35]	1	1	0	1	0	0	1	1	1	6
Turgut [33]	1	1	0	1	1	0	1	1	1	7
Khandhadia [34]	1	0	0	0	0	0	1	1	1	4

‘Y’ denotes ‘yes’ and ‘N’ denotes ‘no’. Quality items: 1) Definition of case; 2) Representativeness of the cases; 3) Selection of Controls; 4) Definition of Controls; 5) Study control for age; 6) Study controls for smoking; 7) Ascertainment of exposure; 8) Same method of ascertainment for cases and controls; 9) Non-response rate.