2.3. Quality Assessment and Data Extraction

The quality of the selected studies was evaluated using the Newcastle-Ottawa Scale (NOS), which consists of three components: selection, comparability, and outcome. Selection included four parts: representativeness of the exposed cohort; selection of the nonexposed cohort; ascertainment of exposure; and outcomes not present at the start of the study. Outcomes included three items: assessment of outcomes; length of follow-up; and adequacy of follow-up. Studies with a total evaluation score ≥7 were of high quality [30].

Data extracted from the selected articles included author, year of publication, country of research, sample size, sex, age, body mass index (BMI) of the study participants, tumor node metastasis stage (TNM stage), metformin therapeutic scheme and mode of administration, interventions in the control group, GC therapy, and type of gastrectomy. Any disagreement between the two investigators was resolved through arbitration by a third investigator.