Table 7.

Quality appraisal OS

Reference	Study addressed a clearly focused issue	Authors used an appropriate method to answer their question	Cases/cohort were/was recruited in an acceptable way	Controls were selected in an acceptable way	The exposure was accurately measured to minimize bias	The outcome was accurately measured to minimize bias	The authors identified all important confounding factors	The follow up of subjects was complete enough	The follow up of subjects was long enough	Results can be applied to the local population
Sam 2004 [59]	✓	✓	✓	✓	✓	✗	✗	✓	✓	✗
Burton 2006 [48]	✓	✓	✓	✓	✓	✗	✗	✓	✓	✓

Note: ✓: yes, ✗: no