Table 4. Summary of bias for the included studies.

Study number	Study design	Sampling bias	Random allocation	Blinding	Comparability of groups	Detection bias	Incomplete outcome data	Selective reporting
1			NA
2			NA
3			NA
4					NA
5					NA
6			NA
7			NA
8			NA
9					NA
10			NA
11			NA
12			NA
13			NA

Red denotes a high risk of bias, yellow, medium, and green, a low risk of bias. NA (not applicable) refers to measures which do not apply to the study due to its design.