Data extraction and quality score assessment
Data was extracted and checked by two investigators independently.
Disagreement was settled through discussion and consensus. The following
information was collected: (1) first author name, (2) year of
publication, (3) country, (4) geographic region, (5) ethnicity, (6)
sample size, (7) source of controls, (8) type of control, (9) matching,
(10) genotypes distribution, (11) location of CRC, (12) site of CRC, and
(12) gender.
Scale for quality assessment criteria was shown in Table 1. All eligible
studies quality was also independently assessed by two authors. Total
scores ranged from 0 (worst) to 21 (best). If scores were < 13
(the eligible studies were regarded as low quality) and If scores of ≥
13 (the eligible studies were regarded as high quality).