Table 2.

Risk assessment of non-RCT study bias.

	Dili et al[18]	Zhang[26]	Chen and Fu[17]	Tang[19]	Liu et al[23]	Li[27]
Representativeness of the exposed cohort	1	1	1	1	1	1
Selection of the nonexposed cohort	1	1	1	1	1	1
Ascertainment of exposure	1	1	1	1	1	1
Demonstration that outcome of interest was not present at start of study	0	0	1	1	1	1
Comparability of cohorts on the basis of the design or analysis	1	1	1	1	1	1
Assessment of outcome	0	0	0	0	0	0
Was follow-up long enough for outcomes to occur	1	1	0	0	1	0
Adequacy of follow-up of cohorts	1	1	0	0	1	0
Total	6	6	5	5	7	5

RCT = randomized controlled trial.