Table 2.

Risk of bias (high risk of bias, low risk of bias, unclear risk of bias). [For prompts indicating high risk of bias see Additional file 3]

Study	1. Clear and focused aim	2. Match between aim and chosen method(s)	3. The best data sources(s)	4. All important variables considered	5. The same variables considered in all sources	6. Data collection is transparent, and data unambiguously identified	7. Classification of the variables unaffected by prior knowledge about the results	8. Appropriate analysis method chosen	9. Systematic error considered	10. Conclusions supported by data
Clarke M, Alderson P, Chalmers I. (2002) [27]	Low risk	Low risk	Unclear	Low risk	Low risk	Low risk	High risk	Low risk	High risk	Low risk
Clarke M, Chalmers I. (1998) [18]	Low risk	Low risk	Unclear	Low risk	Low risk	Low risk	High risk	Low risk	Low risk	Low risk
Clarke M, Hopewell S. (2013) [30]	Low risk	Low risk	Unclear	Low risk	Low risk	Low risk	High risk	Low risk	High risk	Low risk
Clarke M, Hopewell S, Chalmers I. (2007) [28]	Low risk	Low risk	Unclear	Unclear	Low risk	Low risk	High risk	Low risk	High risk	Low risk
Clarke M, Hopewell S, Chalmers I. (2010) [29]	Low risk	Low risk	Unclear	Low risk	Low risk	Low risk	High risk	Low risk	High risk	Low risk
Engelking A, Cavar M, Puljak L. (2018) [26]	Low risk	Low risk	Unclear	Low risk	Low risk	Unclear	High risk	Low risk	Low risk	Low risk
Goudie AC et al. (2010) [31]	Low risk	Low risk	Unclear	Low risk	Low risk	Unclear	High risk	Low risk	Low risk	Low risk
Helfer B et al. (2015) [32]	Low risk	Low risk	Unclear	Low risk	Low risk	Low risk	High risk	Low risk	Low risk	Low risk
Hoderlein X, Moseley AM, Elkins MR. (2017) [20]	Low risk	Low risk	Low risk	Low risk	Low risk	Low risk	High risk	Low risk	Low risk	Low risk
Johnson AL, Walters C, Gray, H et al. (2020) [21]	Low risk	Low risk	Unclear	Low risk	Low risk	Low risk	Low risk	Low risk	Low risk	Low risk
Rauh S, Nigro T, Sims M et al. (2020) [22]	Low risk	Low risk	Unclear	Unclear	Low risk	Low risk	High risk	Low risk	High risk	Low risk
Rosenthal R, Bucher HC, Dwan K. (2017) [23]	Low risk	Low risk	Unclear	Unclear	Low risk	Low risk	High risk	Low risk	High risk	Low risk
Shephard S, Wise A, Johnson BS et al. (2021) [19]	Low risk	Low risk	Low risk	Low risk	Low risk	Low risk	High risk	Low risk	Low risk	Low risk
Torgeson T, Evans S, Johnson BS et al. (2020) [25]	Low risk	Low risk	Unclear	Low risk	Low risk	Low risk	High risk	Low risk	Low risk	Low risk
Walters C, Torgeson T, Fladie I et al. (2020) [24]	Low risk	Low risk	Unclear	Low risk	Low risk	Low risk	High risk	Low risk	High risk	Low risk