Summary of the methodological quality assessment for each study using the Hoy et al19 risk of bias tool
| Study | Q1 | Q2 | Q3 | Q4 | Q5 | Q6 | Q7 | Q8 | Q9 | Total |
| Asghar et al16 | 1 | 1 | 1 | 1 | 0 | 0 | 0 | 0 | 0 | 4 |
| Balbinot et al38 | 0 | 0 | 1 | 1 | 0 | 0 | 0 | 0 | 0 | 2 |
| Barr et al39 | 0 | 0 | 1 | 1 | 0 | 0 | 0 | 0 | 0 | 2 |
| Bongaerts et al24 | 1 | 0 | 1 | 0 | 0 | 0 | 0 | 0 | 0 | 2 |
| Callaghan et al29 | 0 | 1 | 1 | 1 | 0 | 0 | 0 | 0 | 0 | 3 |
| Callaghan et al32 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 1 |
| De Neeling et al20 | 1 | 0 | 0 | 1 | 0 | 0 | 0 | 0 | 0 | 2 |
| Dimova et al25 | 1 | 0 | 1 | 1 | 0 | 0 | 0 | 0 | 0 | 3 |
| Dyck et al14 | 0 | 0 | 0 | 1 | 0 | 0 | 0 | 0 | 0 | 1 |
| Franklin et al28 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 |
| Fujimoto et al12 | 1 | 0 | 1 | 1 | 0 | 0 | 0 | 0 | 0 | 3 |
| Fujimoto et al13 | 1 | 0 | 1 | 1 | 0 | 0 | 0 | 0 | 0 | 3 |
| Gabriel et al37 | 1 | 0 | 0 | 1 | 0 | 0 | 0 | 0 | 0 | 2 |
| Herman et al40 | 0 | 0 | 0 | 1 | 0 | 0 | 0 | 0 | 0 | 1 |
| Kannan et al41 | 0 | 1 | 1 | 1 | 0 | 0 | 0 | 0 | 0 | 3 |
| Kopf et al27 | 1 | 1 | 1 | 0 | 0 | 0 | 0 | 0 | 0 | 3 |
| Kurisu et al36 | 0 | 0 | 1 | 1 | 0 | 0 | 0 | 0 | 0 | 2 |
| Lee et al11 | 0 | 1 | 1 | 0 | 0 | 0 | 0 | 0 | 0 | 2 |
| Lin et al30 | 0 | 0 | 0 | 1 | 0 | 0 | 0 | 0 | 0 | 1 |
| Liu et al33 | 1 | 0 | 1 | 1 | 0 | 0 | 0 | 0 | 0 | 3 |
| Lu et al31 | 0 | 1 | 1 | 1 | 0 | 0 | 0 | 0 | 0 | 3 |
| Németh et al26 | 0 | 0 | 1 | 1 | 0 | 0 | 0 | 0 | 0 | 2 |
| Oohashi et al35 | 0 | 0 | 1 | 1 | 0 | 0 | 1 | 0 | 0 | 3 |
| Saadi et al42 | 0 | 0 | 1 | 1 | 0 | 0 | 0 | 0 | 0 | 2 |
| Sahin et al43 | 0 | 1 | 1 | 1 | 0 | 0 | 0 | 0 | 0 | 3 |
| Zeng et al34 | 1 | 0 | 1 | 1 | 0 | 0 | 0 | 0 | 0 | 3 |
| Ziegler et al21 | 1 | 0 | 0 | 1 | 0 | 0 | 0 | 0 | 0 | 2 |
| Ziegler et al10 | 0 | 0 | 0 | 1 | 0 | 0 | 0 | 0 | 0 | 2 |
| Ziegler et al23 | 1 | 0 | 0 | 1 | 0 | 0 | 0 | 0 | 0 | 2 |
Please see online supplemental appendix 2 for full tool. Q1: Was the study’s target population a close representation of the national population in relation to relevant variables, for example, age, sex, occupation? Q2: Was the sampling frame a true or close representation of the target population? Q3: Was some form of random selection used to select the sample, or was a census undertaken? Q4: Was the likelihood of non-response bias minimal? Q5: Were data collected directly from the subjects (as opposed to a proxy)? Q6: Was an acceptable case definition used in the study? Q7: Was the study instrument that measured the parameter of interest shown to have reliability and validity? Q8: Was the same mode of data collection used for all subjects? Q9: Were the numerator(s) and denominator(s) for the parameter of interest appropriate? Overall risk of bias score: 0–3=low risk, 4–6=moderate risk, 7–9=high risk.