Prognostic value of circulating tumor cells in esophageal cancer

Table 2 Assessment of the risk of bias in each cohort study using the Newcastle-Ottawa scale

Study	Selection				Comparability	Outcome			Totalscore
	Exposed cohort	Non-exposed cohort	Ascertainment of exposure	Outcome of interest		Assessment of outcome	Length of follow-up	Adequacy of follow-up
Li et al[26], 2016	NS	S	S	S	NS	S	S	S	6
Su et al[25], 2016	S	S	S	S	NS	S	S	S	7
Reeh et al[27], 2015	S	S	S	S	NS	S	S	S	7
Matsushita et al[28], 2015	NS	S	S	S	NS	S	S	S	6
Tanaka et al[29], 2015	S	S	S	S	NS	S	NS	S	6
Yin et al[30], 2012	NS	S	S	S	NS	S	NS	S	5
Tanaka et al[31], 2010	NS	S	S	S	NS	S	S	S	6
Hoffmann et al[32], 2010	S	S	S	S	NS	S	S	S	7
Gao et al[33], 2009	NS	S	S	S	NS	S	NS	NS	4

A higher overall score corresponds to a lower risk of bias; a score of ≤ 5 (out of 9) indicates a high risk of bias. S: The study is satisfied the item; NS: The study is not satisfied the item.