Table 2

Quality assessment of included studies

	Q1	Q2	Q3	Q4	Q5	Q6	Q7	Q8	Q9	Q10	Q11	Q12	Q13	Q14	Quality rating (good, fair or poor)
Marcenes and Sheiham27	Yes	Yes	NR	Yes	Yes	No	No	Yes	Yes	No	Yes	Yes	NA	Yes	Fair
Freeman and Goss23	Yes	Yes	NR	No	Yes	No	No	Yes	Yes	No	Yes	Yes	NA	No	Poor
Linden et al28	Yes	Yes	NR	Yes	Yes	No	No	Yes	Yes	No	Yes	Yes	NA	No	Poor
Genco et al29	Yes	Yes	NR	No	Yes	No	No	Yes	No	No	Yes	Yes	NA	No	Poor
Akhter et al30	Yes	Yes	NR	No	Yes	No	No	NA	No	No	Yes	Yes	NA	No	Poor
Talib Bandar22	Yes	Yes	NR	No	No	No	No	NA	No	No	Yes	Yes	NA	No	Poor
Mahendra et al31	Yes	Yes	NR	Yes	Yes	No	No	NA	Yes	No	Yes	Yes	NA	No	Poor
Ramji24	Yes	Yes	No	Yes	Yes	No	No	NA	No	No	Yes	Yes	NA	No	Poor
Islam et al32	Yes	Yes	NR	Yes	Yes	No	No	NA	No	No	Yes	Yes	NA	No	Poor
Hayashi et al33	Yes	Yes	Yes	Yes	Yes	No	No	NA	Yes	No	Yes	Yes	NA	No	Fair
Sato et al34	Yes	Yes	No	Yes	Yes	No	No	NA	Yes	No	Yes	No	NA	Yes	Fair

Q1. Was the research question or objective in this paper clearly stated?
Q2. Was the study population clearly specified and defined?
Q3. Was the participation rate of eligible persons at least 50%?
Q4. Were all the subjects selected or recruited from the same or similar populations (including the same time period)? Were inclusion and exclusion criteria for being in the study prespecified and applied uniformly to all participants?
Q5. Was a sample size justification, power description, or variance and effect estimates provided?
Q6. For the analyses in this paper, were the exposure(s) of interest measured prior to the outcome(s) being measured?
Q7. Was the timeframe sufficient so that one could reasonably expect to see an association between exposure and outcome if it existed?
Q8. For exposures that can vary in amount or level, did the study examine different levels of the exposure as related to the outcome (eg, categories of exposure, or exposure measured as continuous variable)?
Q9. Were the exposure measures (independent variables) clearly defined, valid, reliable and implemented consistently across all study participants?
Q10. Was the exposure(s) assessed more than once over time?
Q11. Were the outcome measures (dependent variables) clearly defined, valid, reliable and implemented consistently across all study participants?
Q12. Were the outcome assessors blinded to the exposure status of participants?
Q13. Was loss to follow-up after baseline 20% or less?
Q14. Were key potential confounding variables measured and adjusted statistically for their impact on the relationship between exposure(s) and outcome(s)?
NA, not applicable; NR, not reported.