Table 2. Newcastle-Ottawa scale scores.
| Study | Selection | Comparability | Outcome | Total |
|---|---|---|---|---|
| Lu 2017 [3] | 4 | 2 | 2 g | 8 |
| Song 2014 [4] | 4 | 1 f | 2 g | 7 |
| Sanchez-Diaz 2014 [5] | 4 | 0 e f | 1 h i | 5 |
| Hong 2014 [41] | 4 | 1 f | 2 g | 7 |
| Yang 2015 [42] | 4 | 1 f | 2 g | 7 |
| Liu 2015 [43] | 4 | 0 e f | 1 h i | 5 |
| Wang 2014 [45] | 4 | 0 e f | 3 | 7 |
| Allen-Rhoades 2015 [44] | 4 | 0 e f | 1 h i | 5 |
| Liu 2015 [46] | 4 | 0 e f | 0 g h i | 4 |
| Han 2015 [47] | 4 | 2 | 2 h | 8 |
| Cai 2015 [48] | 4 | 1 f | 3 | 8 |
| Han 2015 [49] | 4 | 1 f | 2 g | 7 |
| Wang 2016 [50] | 4 | 2 | 1 h i | 7 |
| Xu 2014 [51] | 4 | 1 f | 1 h i | 6 |
| Fei 2014 [52] | 4 | 2 | 1 h i | 7 |
| Yuan 2012 [53] | 4 | 1 f | 1 h i | 6 |
| Ren 2016 [54] | 4 | 1 f | 1 h i | 6 |
| Xu 2014 [55] | 4 | 1 e | 2 g | 7 |
| Xu 2014 [56] | 4 | 2 | 2 g | 8 |
| Sarver 2013 [57] | 4 | 0 e f | 0 g h i | 4 |
| Ma 2014 [58] | 4 | 2 | 2 g | 8 |
| Zhang 2016 [59] | 4 | 0 e f | 0 g h i | 4 |
| Meng 2016 [60] | 4 | 2 | 2 g | 8 |
| Wu 2010 [61] | 4 | 2 | 2 g | 8 |
Reasons for lost stars: a no description of the derivation of the cohort; b no description of the derivation of the non-exposed cohort; c no description of exposure ascertainment; d no description of whether outcome of interest was not present at the start of study; e study not controlling the most important factor such as TNM stage; f study not controlling other additional factors, such as age, gender, and smoke; g no description of outcome assessment; h inadequacy of follow-up of cohorts; I follow-up not long enough for outcomes to occur.