Table 3. Methodological quality score using the CASP scale of relevant studies.

CASP scale	Brown et al (2004)	Brown et al (2008)	Caulfield & Garrett (2004)	Caulfield et al (2004)	Delahunt et al (2006)	De Ridder et al (2015)	Doherty et al (2016)	Lee et al (2017)	Lin et al (2011)	Suda et al (2009)	Zhang et al (2012)
Did the study address a clearly focused issue?	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes
Did the authors use an appropriate method to answer their question?	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes
Were the cases recruited in an acceptable way?	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes
Were the controls selected in an acceptable way?	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	No	Yes	No
Was the exposure accurately measured to minimize bias?	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	CT
Aside from the experimental intervention, were the groups treated equally?	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes
Have the authors taken account of the potential confounding factors in the design and/or in their analysis?	CT	CT	CT	CT	CT	CT	CT	CT	CT	CT	CT
How large was the treatment effect?	CT	CT	CT	CT	CT	CT	CT	CT	CT	CT	CT
How precise was the estimate of the treatment effect?	CT	CT	CT	CT	CT	CT	CT	CT	CT	CT	CT
Do you believe the results?	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes
Can the results be applied to the local population?	Yes	Yes	Yes	Yes	CT	CT	Yes	CT	Yes	Yes	Yes
Do the results of this study fit with other available evidence?	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	CT
Yes score (%) *	9(75)	9(75)	9(75)	9(75)	8(67)	8(67)	9(75)	8(67)	8(67)	9(75)	6(50)
Level of evidence	2	2	2	2	2	2	2	2	2	2	2

* Scores are based on items 1 thorough 12. CASP, Critical Appraisal Skills Programme; CT, cannot tell.