INFLUENCE OF METHODOLOGIC FACTORS IN A POOLED ANALYSIS OF 13 CASE-CONTROL STUDIES OF COLORECTAL-CANCER AND DIETARY FIBER

We examined the study design features and data collection methods from 13 case control studies of colorectal cancer and diet, which had been previously combined and analyzed, to determine whether they influence d the results obtained from a pooled analysis. We assessed the methods used in each study, estimated a quality score, and used random effect s models to re estimate the pooled odds ratio for the association betw een dietary fiber and colorectal cancer for these data. Key features o f the methods used in each study and the quality score were examined i n random effects models to determine whether the heterogeneity found b etween study- specific risk estimates could be explained by these vari ables. The odds ratio for dietary fiber and colorectal cancer was 0.46 (95% confidence interval = 0.34-0.64) for the 13 case-control studies as estimated with a random effects model. Two factors, whether the di et questionnaire had been validated before use in the case control stu dy and whether qualitative data on dietary habits and cooking methods had been incorporated into the nutrient estimation, explained some of the heterogeneity found between studies. Risk estimates for dietary fi ber and colorectal cancer were closer to the null for the studies that had these two characteristics. Quality score did not explain any betw een study heterogeneity. Random effects models, which included fixed e ffects covariates, explained some between study heterogeneity in these data and would be useful for future pooled analyses.