Table 5.

Quality assessment of cohort studies.

Studies	Selection	Comparability	Outcome	Score
Wang[20]	★★★	★	★★	6
Liang et al[21]	★★★★	★	★★★	8
Peng et al[22]	★★★	★	★★	6
Liang[23]	★★★★	★	★★★	8
Chen et al[24]	★★★	★	★★★	7
Yao[25]	★★★★	★	★★★	8
Zhang[27]	★★★★	★	★★★	8
Huet al[28]	★★★★	★	★★★	8

The Cohort studies were evaluated using the Newcastle-Ottawa scale, which are comprised of the study of selection (Representativeness of the exposed group, Representativeness of the non exposed group, Ascertainment of exposure, Demonstration that outcome of interest was not present at start of study), group comparability(Controls for the most important factor, Controls for any additional factor), outcome measures (Assessment of outcome, Was follow-up long enough for outcomes to occur, Adequacy of follow up of cohorts), a total of 9 points. ★, 1 point.