Quality assessment of studies in the meta-analysis based on modified NOS judgment.

Selection: 1. Is the subject definition adequate or described? (if yes, one star); 2. Was the subject representative of the total population? (one star, if truly or obviously; no stars if subjects were selected group or not described). Comparability: 3. Did the study have no differences between laparoscopic and liver resection for CLM? Five main factors were considerate: positive node of primary tumor, disease-free interval, number of liver metastases, presence of liver tumor, CEA level. Other four factors: age, sex, ASA score, and pre- and postoperative chemotherapy were comparative (if yes, two stars; one star if there were no other differences between the two groups even if one or more of these five characteristics was not reported; no star was assigned if the two groups differed). Outcome assessment: 4. Clearly defined outcome of interest (if yes, one star); 5. Adequacy of follow-up (one star if less than 20% of CLM patients lost to follow-up, otherwise no stars).